A GPU is the same thing as a graphics card. GPU = Graphics Processing Unit. Simply depends who you're talking to as to which term they use
As to whether you need one or not, if you're into modern games, CAD, video editing/production, number crunching, etc., then you'll benefit greatly from having one. If your PC will be used for browsing the internet, youtube videos and such, then you can go without one and save the money.