What is the difference between a process and a thread?

Both processes and threads are independent sequences of execution. The typical difference is that threads (of the same process) run in a shared memory space, while processes run in separate memory spaces.

I'm not sure what "hardware" vs "software" threads you might be referring to. Threads are an operating environment feature, rather than a CPU feature (though the CPU typically has operations that make threads efficient).

Erlang uses the term "process" because it does not expose a shared-memory multiprogramming model. Calling them "threads" would imply that they have shared memory.

Process
Each process provides the resources needed to execute a program. A process has a virtual address space, executable code, open handles to system objects, a security context, a unique process identifier, environment variables, a priority class, minimum and maximum working set sizes, and at least one thread of execution. Each process is started with a single thread, often called the primary thread, but can create additional threads from any of its threads.

Thread
A thread is an entity within a process that can be scheduled for execution. All threads of a process share its virtual address space and system resources. In addition, each thread maintains exception handlers, a scheduling priority, thread local storage, a unique thread identifier, and a set of structures the system will use to save the thread context until it is scheduled. The thread context includes the thread's set of machine registers, the kernel stack, a thread environment block, and a user stack in the address space of the thread's process. Threads can also have their own security context, which can be used for impersonating clients.

This information was found on Microsoft Docs here: About Processes and Threads

Microsoft Windows supports preemptive multitasking, which creates the effect of simultaneous execution of multiple threads from multiple processes. On a multiprocessor computer, the system can simultaneously execute as many threads as there are processors on the computer.

Process:

• An executing instance of a program is called a process.
• Some operating systems use the term ‘task‘ to refer to a program that is being executed.
• A process is always stored in the main memory also termed as the primary memory or random access memory.
• Therefore, a process is termed as an active entity. It disappears if the machine is rebooted.
• Several process may be associated with a same program.
• On a multiprocessor system, multiple processes can be executed in parallel.
• On a uni-processor system, though true parallelism is not achieved, a process scheduling algorithm is applied and the processor is scheduled to execute each process one at a time yielding an illusion of concurrency.
• Example: Executing multiple instances of the ‘Calculator’ program. Each of the instances are termed as a process.

Thread:

• A thread is a subset of the process.
• It is termed as a ‘lightweight process’, since it is similar to a real process but executes within the context of a process and shares the same resources allotted to the process by the kernel.
• Usually, a process has only one thread of control – one set of machine instructions executing at a time.
• A process may also be made up of multiple threads of execution that execute instructions concurrently.
• Multiple threads of control can exploit the true parallelism possible on multiprocessor systems.
• On a uni-processor system, a thread scheduling algorithm is applied and the processor is scheduled to run each thread one at a time.
• All the threads running within a process share the same address space, file descriptors, stack and other process related attributes.
• Since the threads of a process share the same memory, synchronizing the access to the shared data within the process gains unprecedented importance.

I borrowed the above info from the Knowledge Quest! blog.

An application consists of one or more processes. A process, in the simplest terms, is an executing program. One or more threads run in the context of the process. A thread is the basic unit to which the operating system allocates processor time. A thread can execute any part of the process code, including parts currently being executed by another thread. A fiber is a unit of execution that must be manually scheduled by the application. Fibers run in the context of the threads that schedule them.

Stolen from here.

A process is a collection of code, memory, data and other resources. A thread is a sequence of code that is executed within the scope of the process. You can (usually) have multiple threads executing concurrently within the same process.

Both threads and processes are atomic units of OS resource allocation (i.e. there is a concurrency model describing how CPU time is divided between them, and the model of owning other OS resources). There is a difference in:

• Shared resources (threads are sharing memory by definition, they do not own anything except stack and local variables; processes could also share memory, but there is a separate mechanism for that, maintained by OS)
• Allocation space (kernel space for processes vs. user space for threads)

Greg Hewgill above was correct about the Erlang meaning of the word "process", and here there's a discussion of why Erlang could do processes lightweight.

Process:

Process is basically a program in execution. It is an active entity. Some operating systems use the term ‘task‘ to refer to a program that is being executed. A process is always stored in the main memory also termed as the primary memory or random access memory. Therefore, a process is termed as an active entity. It disappears if the machine is rebooted. Several process may be associated with a same program. On a multiprocessor system, multiple processes can be executed in parallel. On a uni-processor system, though true parallelism is not achieved, a process scheduling algorithm is applied and the processor is scheduled to execute each process one at a time yielding an illusion of concurrency. Example: Executing multiple instances of the ‘Calculator’ program. Each of the instances are termed as a process.

Thread:

A thread is a subset of the process. It is termed as a ‘lightweight process’, since it is similar to a real process but executes within the context of a process and shares the same resources allotted to the process by the kernel. Usually, a process has only one thread of control – one set of machine instructions executing at a time. A process may also be made up of multiple threads of execution that execute instructions concurrently. Multiple threads of control can exploit the true parallelism possible on multiprocessor systems. On a uni-processor system, a thread scheduling algorithm is applied and the processor is scheduled to run each thread one at a time. All the threads running within a process share the same address space, file descriptors, stack and other process related attributes. Since the threads of a process share the same memory, synchronizing the access to the shared data withing the process gains unprecedented importance.

ref-https://practice.geeksforgeeks.org/problems/difference-between-process-and-thread

Difference between Thread and Process?

A process is an executing instance of an application and A thread is a path of execution within a process. Also, a process can contain multiple threads.It’s important to note that a thread can do anything a process can do. But since a process can consist of multiple threads, a thread could be considered a ‘lightweight’ process. Thus, the essential difference between a thread and a process is the work that each one is used to accomplish. Threads are used for small tasks, whereas processes are used for more ‘heavyweight’ tasks – basically the execution of applications.

Another difference between a thread and a process is that threads within the same process share the same address space, whereas different processes do not. This allows threads to read from and write to the same data structures and variables, and also facilitates communication between threads. Communication between processes – also known as IPC, or inter-process communication – is quite difficult and resource-intensive.

Here’s a summary of the differences between threads and processes:

1. Threads are easier to create than processes since they don't require a separate address space.

2. Multithreading requires careful programming since threads share data strucures that should only be modified by one thread at a time. Unlike threads, processes don't share the same address space.

3. Threads are considered lightweight because they use far less resources than processes.

4. Processes are independent of each other. Threads, since they share the same address space are interdependent, so caution must be taken so that different threads don't step on each other.
   This is really another way of stating #2 above.

5. A process can consist of multiple threads.

I've perused almost all answers there, alas, as an undergraduate student taking OS course currently I can't comprehend thoroughly the two concepts. I mean most of guys read from some OS books the differences i.e. threads are able to access to global variables in the transaction unit since they make use of their process' address space. Yet, the newly question arises why there are processes, cognizantly we know already threads are more lightweight vis-à-vis processes. Let's glance at the following example by making use of the image excerpted from one of the prior answers,

We have 3 threads working at once on a word document e.g. Libre Office. The first does spellchecking by underlining if the word is misspelt. The second takes and prints letters from keyboard. And the last does save document in every short times not to lose the document worked at if something goes wrong. In this case, the 3 threads cannot be 3 processes since they share a common memory which is the address space of their process and thus all have access to the document being edited. So, the road is the word document along with two bulldozers which are the threads though one of them is lack in the image.

Difference between process and thread are given below :

• Process is an executing instance of a program whereas Thread is the smallest unit of process .
• Process can be divided into multiple threads whereas Thread can not be divided.
• Process may be considered as a task whereas Thread may be considered as a task lightweight process.
• Process allocate separate memory space whereas Thread allocate shared memory space.
• Process is maintained by operating system whereas Thread is maintained by programmer.

I believe the easiest way to understand the difference is to visualise how threads and processes execute their jobs.

Threads are running in parallel, in a shared memory space (of the process that created them):

Thread 1              Thread 2              Thread 3
   | 
   | 
   |
                         |
                         |
                                               |
                                               |
                                               |
   |
                         |
                         | 
                         |            
Complete             Complete              Complete

Note: The above can be interpreted as a process (i.e. one process with 3 threads)

Processes are running in parallel and concurrently:

Process 1              Process 2              Process 3
    |                      |                      |
    |                      |                      |
    |                      |                      |
    |                      |                      |
    |                      |                      |
    |                      |                      |
Complete               Complete               Complete

Process - Program in execution

Thread - a thread is execution of the smallest sequence of programmed instructions

Eg- you want to calculate matrix multiplication you will write a program of 3 for loops inside main and execute it . Now this is your process.

Now the same program you can solve by creating threads and assigning each thread to execute result of row. Each thread will work independently and the result will stored in an array. As the threads share the same memory inside a process .

In Both the cases result will be same.

From Erlang Programming (2009): Erlang concurrency is fast and scalable. Its processes are lightweight in that the Erlang virtual machine does not create an OS thread for every created process. They are created, scheduled, and handled in the VM, independent of underlying operating system.

Erlang implements a preemptive scheduler, which allows each process to run for a set period of time without blocking a system thread for too long, which gives each process some cpu time to be executed. The number of system threads depends on the number of cores if I'm not mistaking, and processes can be removed from one thread and moved to another if the load becomes uneven, this is all handled by the Erlang scheduler.

Thread is a light weight process while, the process is a self contained execution environment.

What is meant by "self contained execution process"? Private set of basic runtime resources.

What is meant by "private set of basic runtime resources"? The space allocated from memory to run the the process.(Simply a memory space.)