Although DJ turntables have changed little in the last 20 years, DJ Mixers have evolved considerable.
There is a wide selection of DJ Mixers, each focused to a different user some mixers are for scratching, some for regular mixing, some for touring DJ's
Apart form the core functionality of mixing audio sources, now days DJ mixers have incoporated a lot of extras like loopers, effects and even USB conection so that computers (PC or MAC) can be connected and contrl the music source with the mixer.
The simplest mixer has 2 channels which allow to connect 2 different audio source from CD, MP3 players, computers, Ipods, etc. If you would like to mix several audio sources you would need a DJ micers with more than two channels.
Gain Knobs: Using the LED lights on a mixer as a guide set the gain knob so that it reaches the top of the green but doesn't reach the red. Doing this both audio sources (for example each CD track) will be at the same level.
Equalizer(EQ): Their basic function is to modify the tone They are useful to adjust the tone of individual recordings.. For example to reduce the “brightness” in a song or the increase the bass sound on an specific track.
“Kill” switches: Also known as low pass switch, these complete cut the low frequencies in a track when used. It avoids having to constantly adjust the bass control in the EQ.
Curve controls: It is used to adjust the way a crossfader works
Hamster switch: A hamster switch (named after DJ Hamster) reverses the position of the channels across the crossfader.
Beats Per Minute counters: Basically a LED or liquid crystal display that give an approximate number of beats in a song. Although it may be kind of a novelty they are not very common in the professional high end mixers,
Effects: Instead of having to buy effect modules to hook into the mixer, so high quality mixers have several effects incorporated into their designs. Effect like Delay, Filter, Flanger, Pan & more