The main feature of a mixer are the levels which are clearly shown. Other features include gain, equalisers and reverse. Gain sets the input and output boosts for a channel. Reverse is part of the routing control of a mixing desk.

Bus routing.

Examples of mixing desk manufacturers are mackie, alesis, yamaha. This is a good example set of audio manufacturers
The function of a mixer buses is to give the opportunity for channels/buses to be routed into one channel.  There are several occasions when this might prove to be useful.  There are times when artists require certain track layers to be louder.  A rapper might want all the drum sounds as well as the baseline to be higher than the lead instruments.  You sometimes require the opportunity to make this change without affecting the rest of the mix.  Another occasion why this may prove to be useful is when you want to eliminate a sound altogether or groups of sounds.  He might be playing a bass guitar in a fashion that warrants it to be considered as a percussion instrument.

This is the heart of the mixer.  Generally the left hand side of the board  If you have a 16 channel mixer, you'll obviously have no more than 16 channel strips.  They may be either mono or stereo. Strips are usually a vertical column of controls and consist of most of the following features:
Generally right at the top of the channel strip is your gain or trim control.  This is to properly set the input level for your signal.  There may be other controls like a phase reverse, or low frequency roll off.  
Often, there is an EQ section directly following the mic preamp.  The type of EQ varies from mixer to mixer, but may be parametric or have a fixed frequency.  
Directly after the EQ is usually your Aux sends or Effects sends.  You use these to send a signal to an outboard effects unit.  There will be aux outputs on the back of the mixer to connect to the effects unit.  The higher you turn up the corresponding send, the more signal gets sent to the output.  You use these sends to allow you to send more than one signal to an effects unit.  You may send from as many channels as you want, and the signals will be mixed an sent to the effects unit.    
Some aux sends may be labelled prefader.  the signal is sent to the fx unit at the same volume, regardless of the fader's position.  With a postfader send, lowering the level on the channel's fader will also lower the level going to the aux send.
Subgroup assignment switches.  These switches decide how you want your mix routed through the boards subgroup busses. An obviously, you may pan it anywhere between the extremes to split the signal in any amount to each side.  
The last portion of the channel strip is the fader.  This is the amount of signal that gets sent to the subgroup(s) the channel is assigned and panned to.  Generally a fader's unity gain position is labelled 0dB, and is approximately 3/4 of the way to the top.  You generally place the fader here before setting the input gain. Beside the fader are often two more switches.  These are your mute and solo controls.