In the bible demons are also called evil spirits. Some people believe that these spirits can enter a person's body and take control over it. This is called demonic possession. The normal cure for possession is to expel the evil spirit from the victim's body. This is called casting out the demon. Some people also call it exorcism, and use the name exorcist for anyone who can successfully do it.
The bible describes several cases in which demons are expelled from a possessed person. One example is found at Mark 1:23-26, which says that Jesus cast out a demon from a man in a crowd. There is another example at Luke 8:26-40, which describes how Jesus expelled several demons from a single man. You can find more information about these cases at this demon possession article.
Other biblical passages which mention casting out demons include Luke 8:2-3 and Mark 9:17-30.