There's an app called Activity Monitor that will show all sorts of interesting stats about what the system is doing, otherwise just type 'top -o rsize' in a terminal (without the quotes) and press enter.
Or, in Windows, right click on your task bar and start "Task Manager". The processes tab will tell you what CPU and Memory is in use by what program, and you can click either of those two headings to sort.

Or download the Extended task manager here for even more info....

Extensions for Windows - The Unofficial Windows Upgrade