A projector lamp is a high pressure gas-discharge lamp or a metal-halide lamp. They consist of an arc tube with electrodes, an outer bulb, and a base. Projector lamps produce light by making an electric arc in a mixture of gases. In a metal-halide lamp, the compact arc tube contains a high-pressure mixture of argon, mercury, and a variety of metal halides. The mixture of halides will affect the nature of light produced, influencing the correlated colour temperature and intensity (making the light bluer, or redder, for example).
Metal-halide lamps require electrical ballasts to regulate the arc current and deliver the proper voltage to the arc. Like high-pressure mercury vapour lamps, some metal-halide bulbs contain a third electrode to initiate the arc when the lamp is first lit (which generates a slight flicker when the lamp is first turned on). Pulse-start metal-halide lamps don't contain a starting electrode, but they require an ignitor to generate a high-voltage (1 to 5 kV on cold strike, over 30 kV on hot restrike) pulse to start the arc.
If you want to read more about the technical details of metal-halide lamps please see this Wikipedia page.