UGF means Under Gravel Filter.
This filter provides both mechanical and biological filtration. It is a perforated plate placed just off the bottom of the aquarium tank and then covered in gravel. In the corner of the plate is a tube which sucks up water. This causes the water to be pulled through the gravel, under the plate thus pulling detritus and water through the gravel and into the nitrifying bacteria.
The drawback to this filter is that it can not be dismantled without breaking down the entire tank. It can clog and it is not a good base on which to grow plants as the substrate needs to be quite thin.
This type of filter was once very popular. But now is being slowly dropped in favour for the less troublesome internal or external box filter.