In addition to learning about Chord Extensions, you'll probably also want to start learning about Chord Substitutions. Chord Extensions are additional notes you can add to a chord to give it some new flavor, while Chord Substitutions are completely different chords you can switch in for some of the chords you typically use. Generally, Chord Substitutions will involve chords which use chromatic notes, or in other words, they use notes that don't fall within the key you're playing in, but because of their relationship to the chords being replaced, they still sound good. One common example of a Chord Substitution is the Secondary Dominant.

There's far too much information to provide an adequate posting on a forum, but I highly recommend that you perform a google search on Chord Extensions and Chord Substitutions. You should find adequate resources.