Um… Well, I’ll try to answer anyway.
Exorcism is a formal rite of the Church by which a priest or bishop, and only a priest or bishop, uses the authority of the Church to expel demons from persons and/or things being afflicted by extraordinary demonic activity.
Extraordinary demonic activity comes in several flavors:
Possession, the rarest and most severe
Obsession
Infestation
All extraordinary demonic activity is rare, but many well-known exorcists have asserted that it is becoming more common, relatively speaking.
Ordinary demonic activity, on the other hand, is something which we all suffer. It is commonly experienced as temptation, by which demonic entities try to get us to sin. This does not mean that every time we sin, it is because a demon has tempted us. We can sin regardless, but demonic temptation compounds the matter.