Identifying a running process

[GaryT]

I want to learn how to identify and then stop a process - for example 
today I tried to restart a program that had just crashed and Ubuntu gave 
me a message along the lines of "XYZ is still running but not 
responding. Stop the process or reboot before trying again" or words to 
that effect.

Rebooting was highly undesirable because it meant shutting down other 
programs.  Too bad!

Can someone help save the search and point me to some appropriate 
reading matter?
Much appreciated

GT