a mosfet is short for "metal oxide semiconductor field effect transistor"...................it's basically a transistor that has a higher input impedance than a bipolar transistor....................it also has some other advantages such as no gate current drive, no thermal runaway or second breakdown, low "on resistance" when saturated etc...............also they are faster switches and the drain-source path has no polarity so it can be used in reverse.................those are the basic characteristics..............for more info, u should check a textbook or wikipedia