Difference between PID and TID

Question

What is the difference between PID and TID?

The standard answer would be that PID is for processes while TID is for threads. However, I have seen that some commands

Answer 1

pid and tid are the same except when a process is created with a call to clone with CLONE_THREAD (per the man pages of gettid). In this case, you get a unique thread id but all threads belonging to the same thread group share the same process id.

However, I also recall reading (though I cant find the source) that the values returned from getpid may be cached.

[UPDATE] See the NOTES section here for a discussion on the effects of caching pids.