Mixers are lined up in channels. A channel is a source that is plugged into the mixer like a CD player, turntables or a computer. Controls for a channel are lined up in a column spanning the vertical axis of the mixers. Understand what all the knobs do on one channel and apply that to each different channel, then all those knobs aren't as confusing.
The cross fader is the horizontal slider at the bottom of the mixer in the center. The cross fader's job is to transition sound from channel one to channel two and vice versa. Some mixers have a slope selector which affects how the cross fader transitions the sound between the channels. A cut will transition to full volume quickly. A fade will transition from silent to full volume gradually.
The EQ or equalizer knobs control the different sound frequencies (highs, mids, lows). The EQ gives the mixer tone control. By using the EQ the mixer can cut, attenuate or accentuate specific frequencies on a track. Manipulating the frequency changes how a sound is heard. Most mixers are equipped with a kill switch that cuts out the desired frequency completely.
The gain or trim control can reduce or increase volume separately than the track is currently playing. It is located below the input selector and is used to match signal levels between channels.
Headphones are one of the most important aspects of DJ mixing. Headphones are used to line up songs to be played live. A DJ uses the headphones to listen to other songs and beat match to the song that is being played live. Some mixers offer a "pre-mix" control which allows the DJ to mix queued songs without them being heard live.