When you are playing in a loud club, your best friend is a good pair of headphones. Headphones are the best way to cue up the next song, match its tempo, and prepare to transition to it, without interrupting the main mix. The DJ mixers listed above have a special output called a "cue out" or "monitor out." This output is the DJ's own monitor mix: it allows you to hear the current song while you audition the next track, and sync up the tempo -- providing the audience with a seamless transition.
You'll need headphones to cue up your next track. But what's so special about DJ headphones? For starters, DJ headphones have large, circumaural, over-the-ear earcups to help isolate you from outside noise. Most DJ headphones' earcups swivel 180 degrees, so you can wear them one-sided when you want to hear what the audience hears. Look for features like a detachable, replaceable cable and a 1/4" to 1/8" adapter, and specs like a wide frequency response and high output.