Basicly a sump is an extra body of waterr to your main display tank....
There are many ways of making them work, some have an over flow from the main tank to the sump. then a pump returning the water from the sump to the tank.
others have a pump from the tank to a sump above the main tank that then returns the water via gravity
others are fed by pump both ways.
or you could just divide off the back of a tank as i am planning to do (see
HERE)
they are mainly used for housing the various forms of filtration found in marine tanks. such as RDSB`s (remote deep sand beds) refugiums, miricale mud. aswell as housing skimmers, and heaters and the various other erquipment associated with reef tanks.
the sump can be devided in many ways, with a simple over flow flowing from one side ofr a paine of glass to the other. or an under water weir type thing....
you set the glass and inch or two above the bottom of the sump tank so the water in that compartment has to flow down to the bottom and then up to the top of the next compartment thusly removing any dead spots....
i hope this rambling mess is of some help
Merry crimble