Projection is the display of light onto a surface. There are 2 types of projection used in TV sets; front projection and rear projection.
One drawback of many projection TVs (with the exception of prjection CRTs) is that they use high power lamps. These lamps require replacement, and are often expensive. You should always consider the running costs when looking into a projection based TV.
Front projection is where the light from the projector shines on the front surface of the screen. Front projectors are usually a small box situated 10's of feet from the screen.
Rear projection is where the light from the projector shines onto the rear surface of the viewing screen. Rear projection TVs have the projector built into the TV unit. They are usually bulkier than front projection systems, but do not have the image shining across the room.
There are several projection technologies that are readily available today: