ps will show a list of running processes.
-u: will filter by a user, for example 'root' or 'nobody'
-o: lets you define what columns are output. I use pid, command which gives me the process id, and the name of the command running.
I recently needed to use this because I wanted to stop a memcached daemon from running. I ran this command:
ps -u nobody -o pid,command | grep memcached
to see what the process id of the daemon was. Once I had this, i could do
to kill the process