A three-way light switch allows the light to be controlled from two locations. How does this work?
There might be some confusion because the type of switch you need (single-pole, double-throw) is usually called a three-way switch in the US, and a two-way switch in the UK. So for two switches to control the same lights, you need two SPDT switches, and you can find the wiring diagram on Wikipedia.
Between the two switches is another cable wire connecting both switches. then they both work like train track switches alternating the on/off current to the fixture.