A railgun is a form of electromagnetic weaponry that works on Lorentz forces, magnetic fields created by running power through metal rails connected by a metal projectile. Commonly, video games use gauss guns and call them railguns, for example Quake 2 and 3. Wikipedia has an article on railguns.
As stated, railguns utilize an electromagnetic force called the Lorentz force to propel an electrically conductive projectile that is initially part of the current path. The current flowing through the rails sets up a magnetic field between them and through the projectile perpendicularly to the current in the rail. In short, the rails and the projectile push away from each other, and you wind up throwing a hunk of metal away from wherever the battery is connected.
Railguns have an inherent problem with the amount of power driving the projectile; the rails are forced apart, electrical arcing occurs, and the rails are partially vaporized. Because of this, most railguns can only be fired once before the rails need to be replaced. Our model will be a simple, low-power model designed to throw a projectile, but not do anything particularly spectacular like vaporize a car on impact.