A music mixer can vary in size and in amount of channels, with various built-in functions (such as equalization) and effects (such as reverb and delay). Each individual channel usually has its own basic equalization and both a gain knob and a volume slider that can be used to balance the volume when the individual tracks are combined.
Another key feature of a music mixer is the input for the individual channel. Most mixers have both a line input and an XLR input. The line input is a basic quarter-inch cable input optimized for line level instruments such as a direct box or an output from an audio device. An XLR input is for a microphone cable or any other XLR cable usage. XLR cables can reproduce signals over a longer length of cable, and these types of inputs are preferred over line inputs due to the lack of signal degradation.
Music mixers are very useful in live band or orchestra settings where the music on stage needs to be reproduced for large audiences. After plugging in all of the inputs and cable from the individual microphones and instruments, you can adjust them and add effects to varying degrees. You can also add compression to individual instruments and control what the audience will hear from the stage.
A music mixer can also be used effectively in a recording session. You can use it to record the many pieces of a drum set individually and adjust each piece's volume. You can also record multiple performers at once to give your tracks a live band sound.
Software-based music mixers are also useful in the same way a stand-alone mixer is by providing you a way to control the volume of individual tracks. They even appear similar to their real-world counterparts, but are usually included with your audio production software and use a breakout box or sound card for the inputs.