well, i use them in scool at the physichs lab. it is rated 1000W.
basicly is a wire turned around a metal tube, and insulated from it. it is about half meter long. i has a cursor that mooves around the wire to adjust the resistance.
and there is another kind wich is for lower current, i don't remember exactly how much, but it is a 50 cm wire, connected at both ends to some contacts, and cursor wich you can place somewhere on the wire.
but both types have 3 connections, 2 ends of the wire, wich forms a fixed resistor, and one connections, wich is the cursor.
but, i repeat again, i might have a problem with the dictionary......