I'm assuming you mean a hang on back overflow. The concept is pretty simple, really.
There are two boxes, one on the inside of the tank and the other on the back of the tank.
The box on the inside of the tank has teeth or some other form of skimming/protection from creatures at the top of it (sticking out of the water). Water is skimmed through these teeth and into the inside of the box.
There is a siphon tube that connects the inside box to the box on the outside. Once the siphon is started, this tube carries water from the inside box to the outside box.
Usually the outside box has a little compartment to hold water before it spills over to be drained down the tube. This is so the siphon tube can remain completely submerged under water and keep the siphon going.
Then as the water fills the outside overflow box there are a variety of ways you can send it down the drain to your sump. Some use a standing pipe, various pipe mods, some use prefilter foam.
And for a diagram i did a quick google and found this:
How Does a Overflow box work
It has an explanation as well.