The DJ mixer is the heart any hardware DJ setup (software or hybrid setups may vary) as it allows the DJ to seamlessly crossfade from one track to another which is one of the main points that distinguishes a DJ from merely someone playing music.
A typical hardware setup will have two main sources (vinyl, CD or sometimes iPod), a microphone and maybe an additional sound source such as a DJ laptop, sampler etc.. The DJ mixer needs to be able to mix these different sources effectively.
A DJ mixer can be used in the whole range of DJing from simply beat-mixing one song into the next all the way to creative “turntablism” which uses the DJ mixer and decks as creative sound sources with techniques like scratching, beat-juggling and other creative DJing methods.
Different methods need a different type of mixer though, for example complex scratching needs very short crossfader curves to be able to work quickly enough. So different mixers can be aimed towards different specialities or be more general DJ mixers used for more typical DJing.
The more channels your mixer has, the more different sources it can easily mix together. Each channel will have one or more audio input (sometimes switchable) and usually some EQ (bass and treble or more bands). Sometimes channels will have frequency “kill” switches which can for example be used to kill the bass line on one track so that it mixes better, or for special effects.
DJ mixers also have “cue” functions which enable the DJ to listen to a track on headphones without sending it to the main output. This essential feature enables the DJ to pre-listen in order to choose tracks and to beat-mix effectively.
The other essential feature of a DJ mixer, which like cueing is absent on a non-DJ audio mixer, is the crossfader which enables the DJ to seamlessly change from one track to another in one movement. Some crossfaders have a choice of different curves sometimes aimed at specialist functions such as scratching (which needs extra-short curves).
When choosing a DJ mixer keep in mind the kind of music you’ll be DJing and the kind of sound systems you’ll be playing through and choose the quality of the mixer accordingly.
A better quality DJ mixer will sound better, produce less unwanted hum or hiss, offer better features that make the DJ sound better, and be more reliable and long-lasting. Also compare additional features that could help you but be careful that they will actually help rather than hinder or be a waste of money. For example a good quality BPM counter can really help you beat-mix, but a cheap one will be worse than nothing! Effects can be useful for some styles of DJing but of no use in others styles.