Dissonant chords are built of tones that are not quite in harmony. It's hard to explain since I'm not a music theory teacher, but consonant chords normally sound "good" and dissonant chords sound "bad" or just a little unsettling like I said before. Found a wiki article that explains it a lot better than I can.
http://en.wikipedia.org/wiki/Consonance_and_dissonance
http://en.wikipedia.org/wiki/Consonance_and_dissonance
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