Every wonder why your Mac is running slowly or why you can't get an application to close. Select command+spacebar to search Activity Monitor in spotlight.
The Activity Monitor will tell you what percentage of your CPU an app is using and if an app is unresponsive. Apps that are taking up a significant percentage of your CPU could be slowing your computer down, even if you are not using them/they are running in the background.
You can quit a process/app by selecting iTunes for example and selecting the X on the top left.
You can also go to the network tab and see which apps are using up your bandwidth.