"Harmonic Mixing is an advanced technique used by top DJs all over the world. By mixing tracks that are in the same or related keys, harmonic mixing enables long blends and mash-ups. The goal is to eliminate key clashes." -- from harmonic-mixing.com
Harmonic mixing is obviously the shit right now, especially since you can key your songs automatically with Mixed In Key, a PC/Mac program for $58. The great news is that DJ notation was developed with harmonic mixing in mind.
Without harmonic mixing, mosts sets look like this:
In other words, the DJ can only mix beats to beats, beats to drone or beats to riff. If he tried to mix drone to drone, for example, chances are the resulting sound... would suck. It would be disharmonious.
But when the DJ knows that the two tracks have the same key (= are compatible), he can mix drone to drone, melody to drone or even riff to riff.
Look at this song listing to see how you can leverage both notations (Camelot wheel and DJ notation, that is):