There are three types of plungers:
The Cup Plunger:
The Flange Plunger:
the flange plunger is best for toilets, as it creates a good seal around the curved drain hole. The flange can be turned inside the bell and be used as a cup plunger as well (just make sure to wash the sink afterwards. you DO know where that thing has been.)
The Accordion Plunger:
just like the flange plunger, the accordion plunger is optimal for toilets. The accordion pushes air into the drain and pushes out the clog. A bit more difficult to operate than a flange plunger, it is also made of plastic and can scratch the finish on the toilet as well.
For tips on how to use a plunger, see our follow-up post: How-to: Use a Plunger