What are dreams and what is known about the significance of their content?

Dreams are conventionally defined as mental experiences during REM sleep that are often based in vivid imagery, have a story-like quality, are often bizarre, and seem real to the dreamer. Many recent theorists have, however, questioned almost every aspect of this definition. We know now that people dream in non-REM too, although the dreams are less story-like and vivid in imagery. Also, recent research shows that content is usually not bizarre, and that dreamers are often aware that they are dreaming. Thus, the scientific conceptualisation of dreaming is changing.
Research shows that the content of dreams is usually familiar. Common themes in dreams include things like falling, being pursued, trying repeatedly to do something, school, sex, being late, eating, and being frightened, etc.