A fixed projector screen is a matte white screen with a black border. The screen is sized according to most film standards (typically a 16:9 ratio) and is hung wherever you would like to watch your films. It is not like roll-up projector screens, which curl up, but instead remains fixed on the wall. Wall screens, however, are even simpler and are just a blank wall that has been painted matte white.
Painted wall screens are significantly cheaper than fixed screens. Some may require no money at all if your walls are already matte white. Others can either be painted with simple interior paint or with special paint designed for movie screens.
Walls, with drywall in particular, are textured. These textures may cause the image projected on the wall to appear grainy, muddled or blurry. Every tiny imperfection in the wall, whether it be a bump or streak of runny paint, will detract from the image quality. Painted walls also do not have black borders, which enhance the image quality by sharpening the edges of the frame. Some people will simply paint their own black border on the wall. However, it may be so difficult to paint a border to the perfect screen ratio that it may be worth it to just purchase a screen.
Fixed screens have black borders, which keep the image crisp. A screen with no border, like most wall screens, makes the image fade along the edges of the frame, which may be distracting during a film. Fixed screens are usually manufactured so that they have no texture on the surface, meaning the smoothest image possible will be projected.
Fixed screens are the more expensive of the two options. They must be constructed from various materials, unlike painted walls, which just require a quick coat of white paint. The size of the image must also remain the same, so you can't simply move your projector back if you want a larger image like with a wall screen. There is also the hassle of hanging the screens, an obstacle not present with painted wall screens.