All modern operating systems are able to run many programs at the same time. For example, a typical Linux server might include a Web server, an email server, and probably a database service. Each of these programs runs as a separate process. What do you do if one of your services stops working? Here are some handy command-line tools for managing processes:
Frequently, I'll have Firefox freezing on me and using the "force quit" function only makes it disappear from the GUI - it's still running in the background.
What I usually do is access a terminal to kill the application.