A crossfader is a slider on a dj mixer that performs smooth transitions between an audio input on one channel and an audio input on another channel. If you have two songs playing into the two seperate channels, and the crossfader is in the middle, then both will be playing at the same level, modified only by the volume sliders. If you move the crossfader towards one channel, it will become more prominent as the other dies out, and vice versa. Some mixers include a hamster switch which will reverse the crossfader with one touch.

There are plenty of options to your run-of-the-mill crossfader. (if you don't know what a crossfader is, then the writeup above this was eaten. /msg me so I can fix it.)

1. Adjustable/selectable curve:

   A typical crossfader on a dance DJ mixer has a curve that looks something like two parabolas crossing. When the crossfader is in the middle, neither channel is quite at full volume. A scratch mixer will have that looks more like this:

    _______
   /       \
   

   where each of the side table-leg looking things is the level of one channel. Some cheaper mixers, like the Gemini PMX-60, allow you to select from two or three different curves. Crossfaders on high-end DJ mixers like Ranes, Pioneers, and Allen & Heath mixers have knobs that make a smooth transition from one curve to the next.

2. Beat lights:

   These are little LEDs on each side of the crossfader that light up whenever the corresponding channel's volume change (dV/dt) goes above a certain threshold.

3. Crossfader assign:

   This is handy on mixers with more than two channels. On each end of the crossfader, there is a knob that selects any one of the available channels. The selected channel behaves as it would on a two channel mixer, and the remaining channels bypass the crossfader completely. If the same channel is selected on each end of the crossfader, you will only hear it if the fader is in the MIDDLE.

4. Crossfader disable:

   This makes the channel selected in crossfader assign bypass the crossfader.