Crossfading
A crossfader on a DJ mixer essentially functions like two faders connected side-by-side, but in opposite directions. It allows a DJ to fade one source out while fading another source in at the same time. This is extremely useful when beatmatching two sources of audio (or more, where channels can be mapped to one of the two sides of the crossfader individually) such as phonograph records, compact discs or digital sources.
The technique of crossfading is also used in audio engineering as a mixing technique, particularly with instrumental solos. A mix engineer will often record two or more takes of a vocal or instrumental part and create a final version which is a composite of the best passages of these takes by crossfading between each track.
In the perfect case the crossfader would keep constant output level. However, there's no standard on how this should be achieved. Many DJ equipment manufacturers offer different mixers for different purposes (e.g. scratching, beatmixing, cut mixing, etc.). High-end mixers often have crossfade curve switches allowing the DJ to select the type of crossfade necessary. Experienced DJs are also able to crossfade between tracks using the channel faders.
There are many software applications that feature virtual crossfaders. For instance, burning-software for the recording of audio-CDs.
Read more about this topic: Fade (audio Engineering)