Is there a difference between YES/NO,TRUE/FALSE and true/false in objective-c?

There is no practical difference provided you use BOOL variables as booleans. C processes boolean expressions based on whether they evaluate to 0 or not 0. So:

if(someVar ) { ... }
if(!someVar) { ... }

means the same as

if(someVar!=0) { ... }
if(someVar==0) { ... }

which is why you can evaluate any primitive type or expression as a boolean test (including, e.g. pointers). Note that you should do the former, not the latter.

Note that there is a difference if you assign obtuse values to a so-called BOOL variable and test for specific values, so always use them as booleans and only assign them from their #define values.

Importantly, never test booleans using a character comparison -- it's not only risky because someVar could be assigned a non-zero value which is not YES, but, in my opinion more importantly, it fails to express the intent correctly:

if(someVar==YES) { ... } // don't do this!
if(someVar==NO ) { ... } // don't do this either!

In other words, use constructs as they are intended and documented to be used and you'll spare yourself from a world of hurt in C.

You might want to read the answers to this question. In summary, in Objective-C (from the definition in objc.h):

typedef signed char        BOOL; 
// BOOL is explicitly signed so @encode(BOOL) == "c" rather than "C" 
// even if -funsigned-char is used.
#define OBJC_BOOL_DEFINED


#define YES             (BOOL)1
#define NO              (BOOL)0

I think they add YES/NO to be more self-explanatory in many cases. For example:

[button setHidden:YES];

sounds better than

[button setHidden:TRUE];

No, YES/NO is a different way to refer to TRUE/FALSE(1/0)