There are two basic types of plungers. The "cup" plunger and the "flange" type plunger. The cup plunger is what most people have in their homes and is designed for creating a seal against a flat surface such as the bottom of a sink or bathtub.
Toilets however have a bowl shape and cup plungers don't work because they lose their seal when you pull up. For use with toilets, you need the Flange Plunger.