Activity Monitor is a little know gem of an app hidden in the Utilities folder in your Applications folder.
It can look daunting when you first launch it but once you look past the information overload and find out what to focus on it can become an invaluable tool to make your Mac run faster and smoother.
Find yourself staring at a spinning beach-ball cursor regularly?
Do you hear your computer fan start up regularly?
Are some apps slower than usual?
Activity Monitor will tell you why and help you fix some of those problems.