A mixer can be thought of to have multiple channels. The aim of the mixer is to be able to mix multiple channels together. A Channel can correspond to a music source and controls for each channel runs down the mixer in a parallel fashion. all independent channel usually have similar if not the exact same controls on each channel strip. This means that if you understand one channel strip, you understand them all.
When you get confused by the massive amounts of knobs and fader on a console, rest assured that they are all essentially the same controls over different channels. Whenever you approach a new mixing console, you must first seek to understand the Channel strip. You generally read the channel strip from top to bottom and once you understand one of them, you understand them all. A DJ Mixer will usually have 2 channels. Allowing you to mix 2 sources of music. It is not uncommon to find 4 or more channels on a mixer to have 4 or more music sources giving the user more options.
A 4 channel mixer could, for example, have 2 turntables an ipod and a phone all connected to the mixer. This means that you could mix and match the music sources and can also act as a backup source if one of your turntables becomes faulty.