The PID (process ID) is an acronym for Process identifier. It is nothing but a number used by Linux / Unix kernels to identify a process on the system. Usually, new processes are created using the fork() system call.
How do I find the process ID of a running program?
You need to use the pidof command as follows to find pid of httpd program:
pidof progra-name pidof httpd pidof sshd pidof firefox-bin
You can type the following command to verify that the PID 2582 belongs to firefox:
ls -l /proc/2582/exe
lrwxrwxrwx 1 vivek vivek 0 2010-12-21 22:24 /proc/2582/exe -> /usr/lib/firefox-3.6.13/firefox-bin