The top command is a utility to see what processes and resources are being used on a system in real time via a terminal. Personally I use it constantly to check on the health of my web server when connecting remotely.
The Geek Stuff has a writeup: Can You Top This? 15 Practical Linux Top Command Examples on 15 great things you can do with the top command.
Even though I use this command all the time I seldom use these more advanced features, but I will certainly keep the article around for reference.