The cathode ray tube (CRT) was the traditional technology behind standard- and ultimately high-definition HDTV television sets. CRT technology was found in rear-projection sets and direct view televisions, as well as front projectors.
Even though the image quality was excellent, CRTs were bulky, heavy, complex, and only the most expensive could do a full resolution 1080i signal. They also weren't very bright. CRT front projectors, though lauded for their black levels, produced an extremely dim image on even small-ish projection screens. The current generation of LCOS front projectors offer near-CRT black levels, but much higher brightness.
In an attempt to create as sharp an image as possible give the poor 480i sources of the day, most CRT projectors were paired with line doublers and line quadruplers, the processors of modern-day scalers. The most famous was Faroudja's VP400, which at nearly $30,000 retail was an amazingly expensive component for its era (late 1990s), but when paired with a nine-inch CRT projector, like the Vidikron Vision One, made one heck of a video image at the time.
In the home, LCD and Plasma flat panels have completely replaced direct-view CRTs in the US market.
Check out what the die-hard CRT enthusiasts are saying over at Home Theater Spot.
For one of the last reviews of a CRT front projector, check out this review of a Faroudja 9-inch CRT Video Projector.
For more on how CRTs work, check out the Wikipedia page.