The "sus" means suspended. A major or minor chord has a third (either a major third, or a minor third). The third note is generally the note that gives the chord it's character... The suspended 4th chords do not have a third note, instead they have a 4th. Sus2 is the same thing except you have a 2nd instead of a 4th... Hope that helps you out some. Generally I use them as what I call "passing" chords. In a progression I will throw a few in to add some charcter to the progression... I don't usually base a song around suspended chords. I'm sure you could if you wanted to though.