Projector screens are generally divided into two types base on their functionalities: reflection projector screen and transmission projector screen. It can be also divided into soft and hard screen base on materials they are made from.
Home theater generally uses soft reflection screen. My brother-in-law originally wanted to buy a ¥1000 (~$150) so-called "import screen", but a friend of his who sells projector screens told him that it is hard nowadays (in China) to distinguish genuineness of an import screen, it is hard even for himself. Some of them that are labeled with 'import' or 'joint capital' were actually manufactured somewhere in south of China. He felt that he'd rather to buy a ¥300 domestically manufactured screen with good feelings than buy this kind of "import screen". What this friend said makes perfect sense. But after doing some research, my brother-in-law found that all screens on local market are made from high gain Bolivian bead that is used for projecting newspaper clips, they are simply not suitable for video frequency.
Theoretically speaking, a white wall with one smooth side actually is best "screen". Because its gain is 1, meaning that light projected can be completely reflected out, which is an ideal state of being "no absorption, no gain". Unfortunately, for purpose of absorbing and proliferating sound wave, he already made wall a background wall with sound-absorbing material and plywood installed. making it impossible to serve as a "projector screen', he had to find another solution.
You might be wondering at this point: why do people still bother purchasing expensive screens if we can all use white walls?
Well, there are always benefits and advantages of using a professional screen: convenient, artistically beautiful and dignified, good screen can also make up insufficiency of a projector and improve visual effect. Among expensive screens, one type is "gray screen" (cost about ¥15,000, roughly $2000). This kind of screen probably was originally designed for liquid crystal projectors. The biggest problem with liquid crystal projector is that color appears dark and grey, insufficiently calm. This is its "congenital defect" that is caused by its liquid crystal board and path of rays.
Regarding gray screen, we all know that gray is merely a lighter black, and black absorbs all visible light. Gray can only partially absorb visible light, it is like brightness of picture is reduced. If you have used any picture processing software's "brightness / contrast gradient" option, you should certainly have noticed such phenomenon that reducing brightness is equivalent to increasing contrast gradient? Same concept, since brightness has been reduced, it in turn increased its contrast gradient. The black effect gets improved due to bigger contrast. We can also experience same effect when we look out through sunshade glass of our car. In fact, there are many ways to just reduce brightness, you don't have to use gray screen. There are magazines recommending putting light gray filter of a photographic camera to projection lens, principle is same. You can even use more simpler method, namely you need to adjust projector's output brightness or increase contrast gradient. No need to spend a cent, you may achieve similar effect, but premise is that showroom must be dark enough.