A semiconductor laser is one in which the excited medium is a semiconductor. Pumping (excitation) is obtained by passing an electric current. An optical cavity is not required. The result is a very compact laser (the emitting part is of the order of a micron) whichis cheap to manufacture and easy to control using an electric current.
It is the commonest type of laser and is found in DVD players, printers and pointers. Note semiconductor lasers are also used as exciters (referred to as diode pumping) in ultrashort pulse lasers (of the order of a femtosecond, 10-15s).