An RC flight simulator is a computer program that allows pilots of radio-controlled aircraft to practice on a computer, without the risk and expense of damaging a real model.
Besides the obvious use of training beginners, they are also used for practising new aerobatics, evaluating a model before buying it, and to allow flight practice when conditions are otherwise unsuitable.
There are a number of commercial packages available, such as Eiperle CGM's neXt - RC Flight Simulator, SVK Systems' ClearView, Knife Edge Software's RealFlight, IPACS' AeroFly and Trasna Technology's AccuRC.
There are also a number of free simulators, with SeligSIM (FS One) being the most well known and most feature rich.
There are many web sites dedicated to offering free content like this.