An electrically heated element, under the plate causes the heat, as the current flows though it, there is resistance in the metal heating element, causing the metal to get hot, and consequentially, heating the plate.
If a solution be placed in a beaker or other heat resistant container, then the heat passes by conduction through the containing vessel and heats the solution.
Edit: looks like you got there as I was typing my post Mitch
I want a hotplate badly myself, anyone know of a UK supplier, they HAVE to take solo card though, or else I can't order anything