The top Command Tells You What Your Computer Is Busy With

If you want to find out which programs are using most of your computer's CPU time or most of its memory, simply type in "top" in a terminal window. It will give you a list of processes ordered by the percentage of CPU time they are using. You can change the ordering to memory usage by hitting the "m" key on your keyboard. Of course the top command has a lot more functions and options, which you can find on its man page.