The RC Quadcopter, also called the Quad-copter, quad rotor RC helicopter, or multi-rotor is the latest development in radio controlled vertical lift platforms that are able to take off vertically, hover, and fly in all directions. As the name suggests, there are 4 propellers arranged in a cross type configuration.
There is no single large collective pitch rotor or tail rotor that we are all used to seeing on a conventional RC helicopter. These propellers are fixed pitch, two will spin clockwise and the other two will spin counter clockwise. By precisely and accurately spinning these four propellers at different speeds, all the common directional movements of a standard helicopter are attainable - Hover, forward/backward movement, left/right movement, and yaw (turn rate) movement.
There are certainly other incarnations of the Quadcopter design using as little as 3 propellers arranged in a triangle configuration up to six arranged in a hexagonal configuration and even eight (Octocopters); but for ease of explanation, I will stick with the quad variety as it is the most common and if you do end up getting this type of RC helicopter, most likely what you will be starting out with.