From LQWiki

A pid, or process identifier, is an integer or numeric number used by Unix and other Unix-like operating system kernels, such as Linux, to identify a running process.

Processes are managed by their PIDs - for example, a PID is returned by the fork system call and can be passed to wait() or kill() to perform actions on the given process.

See also kill, killall, ps, pstree, pgrep, proc