"Can any body tell me what is the exact difference between port and pin"

I believe the intended question really is:

What is the difference between a port latch and a port pin?

The answer is:

All instructions that only write to a port actually write to the latch. On the next machine cycle the contents of the latch are copied to the pins.

All instructions that only read a port read the value present on the pins.

Instructions that both read and write from/to a port read from and write to the latch. These are known as the 'read-modify-write' instructions.

It is not entirely obvious which instructions fall into the 'read-modify-write' category. You need to consult the documents cited by others in this thread for a clear explanation.