A process is a running program. Each process on the system has a numeric process ID (PID). For a quick listing of the processes that you're running, just run the ps command on the command line. You should get a list like this:
PID TTY STAT TIME COMMAND 520 p0 S 0:00 -bash 545 ? S 3:59 /usr/X11R6/bin/ctwm -W 548 ? S 0:10 xclock -geometry -0-0 2159 pd SW 0:00 /usr/bin/vi lib/addresses 31956 p3 R 0:00 ps
This is an abridged listing; yours will be much longer if you're running a windowing system. The fields are as follows:
PID The process ID.
TTY The terminal device where the process is running (don't worry about this for now).
STAT The process status; that is, what the process is doing at the given time and where its memory resides. For example, ...