The ceilings of many homes and apartments are not painted, but instead are coated with a substance that has a bumpy texture. It looks like this:

This treatment is normally called a textured finish or an accoustic finish by builders, but most people refer to it as popcorn. This sort of ceiling treatment is popular for several reasons:

  • It keeps you from having to plaster and sand the ceiling - if you've ever done it you know that plastering a ceiling is no fun.
  • It hides lots of imperfections in the ceiling. The ceiling is a huge, flat, uninterrupted and well-lit expanse. Any imperfection is immediately obvious. The texturing hides imperfections very effectively.
  • It helps eliminate echo in a room. If you have ever talked in a room before and after carpeting, you know what a big difference carpet makes on echoes. An acoustic finish is like carpeting the ceiling.
There are many different ways to put a texture on a ceiling, but the most common technique involves mixing some sort of lumpy aggregate - either vermiculite or polystyrene - with ceiling paint. The mixture is then sprayed onto the ceiling using a special spray gun like this hopper texture gun. It goes on in one coat and once it dries (like normal paint) you're done!

Here are several interesting links: