pid：
getpid 获取进程ID
tid：
gettid：取得线程ID，如果是进程，等于 getpid
gid：
getgid ： user group id
引用 stackoverflow
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 atid will automatically work with a pid.
It is worth noting that many functions/system calls/command line utilities documented to work with pidactually use tids. But if the effect is process-wide you will simply not notice the difference.
原文：http://stackoverflow.com/questions/4517301/difference-between-pid-and-tid
另外：
gettid是内核中的线程的ID， pthread_self是POSIX thread ID