typedef syntax clarification

typedef struct {
  int a;
  int b;
} example;

This one defines an unnamed structure type and introduces example as a type alias for that structure type. You can therefore refer to that structure type only as `example.

struct something {
  int a;
  int b;
} ob;

This one defines a structure type something and also declares an object ob of that type. You can refer to the structure type only as struct something.

typedef struct foo {
  int a;
  int b;
} bar;

This one defines a structure type named foo and introduces bar as a type alias for that structure type. You can refer to that structure type as struct foo or as bar.

With

typedef struct {
  int a;
  int b;
} example;

you define an unnamed structure, but define a type-alias example for the structure. That means you can only create instance of the structures using the example "type", like e.g.

example my_example_structure;

With

struct something {
  int a;
  int b;
} ob;

you define a structure named something, and an instance (a variable) of that structure named ob. You can use struct something to create new variables of the structure:

struct something my_second_ob;

The variable ob can be used like any other instance of the structure:

printf("b = %d\n", ob.b);

Lastly, with

typedef struct foo {
  int a;
  int b;
} bar;

you define a structure named foo, so you can use e.g. struct foo to define variables. You also define a type-alias bar which can also be used. Like for example

struct foo my_first_foo;
bar my_second_foo;

The general syntax for typedef is

typedef <actual type> <alias name>;

In the last case of your examples, the <actual type> is

struct foo {
  int a;
  int b;
}

and the <alias name is bar.