Watch the Slash, Add, and Sus Chords online guitar lesson by Jeff Scheetz from Street Theory for Guitarists
Slash chords are pretty useful if you want to mix up the sound of simple chords. The letter on the left of the slash is the chord and the letter on the right of the slash is the bass note.
Add chords are created by taking a triad and "adding" a note in there. Sus chords are made by replacing the 3rd of the chord with a 2nd or a 4th.