Magnetic Stirrer:

Magnetic stir bars are magnets coated in Teflon or some other unreactive polymer. This image shows a star shaped stir bar. They come in variety of shapes and sizes, the most common being rectangular shaped.

Magnetic stir plates have a rotating magnet or rating magnetic field induced by coils inside the plate.

The stir bar is placed in a reaction flask with liquids or solids and placed on top of the stir plate, which stirs the reaction throughout the reaction mixture.

Many stir plates are a combination of both stir plates and heating elements. In this case they will have two control knobs: one which controls the rate of stirring and one that controls the rate of heating.