Fan heaters are popular because they are relatively inexpensive and very portable. Cool air from the room is blown over a heated element by a fan, with the heated air then blown back into the room. This makes them good for spot heating, where heat is needed in a specific place, like being blown in your direction as you sit and watch TV.
For small-to-medium rooms, tower fan heaters are a good idea. Some even allow you to twist the bottom half to face in a different direction to the top half, allowing both sides of the room to be warmed simultaneously.