Difference between PID and TID

It is complicated: pid is process identifier; tid is thread identifier.

But as it happens, the kernel doesn't make a real distinction between them: threads are just like processes but they share some things (memory, fds...) with other instances of the same group.

So, a tid is actually the identifier of the schedulable object in the kernel (thread), while the pid is the identifier of the group of schedulable objects that share memory and fds (process).

But to make things more interesting, when a process has only one thread (the initial situation and in the good old times the only one) the pid and the tid are always the same. So any function that works with a tid will automatically work with a pid.

It is worth noting that many functions/system calls/command line utilities documented to work with pid actually use tids. But if the effect is process-wide you will simply not notice the difference.

Actually, each thread in a Linux process is Light Weight Process (LWP). So, people may call thread as a process... But there is surely a difference.Each thread in a process has a different thread ID (TID) and share the same process ID (PID).

If you are working with pthread library functions, then these functions don't use these TIDs because these are kernel/OS level thread IDs.

Just to add to other answers, according to man gettid:

The thread ID returned by this call is not the same thing as a POSIX thread ID (i.e., the opaque value returned by pthread_self(3)).

So there are two different things one could mean by TID!