Three way switches allow you to turn on a light from two different locations.

Combining four-way switches with two three-way switches gives you the ability to turn on a light from as many places as you need.

A single pole switch only has two terminals. Power goes into one terminal and out the other and to the fixture whenever the switch is turned on. From the fixture the current flows through the bulb to the neutral wire.

A three-way switch has three terminals . There is a marked terminal on most three way switches. It is a different color than the other two. The power or the wire to the fixture goes on this terminal. The other two terminal are for the traveler wires, these are the wires that carry the power from one switch to the other depending on which way the three-way is switched.

A four-way switch has four terminals. These terminals are used for the traveler wires between two three-way switches. You can put as many four-ways between two three-ways as you need.

Power to operate your lights and receptacles comes from you main breaker box through your house wires, a white, a black and a bare ground wire unless you have an older house that was wired before circuits were grounded. The black is the power wire or hot wire it has 110 to 120 volts when measured to ground or neutral.

The white wire is the neutral wire, which means there is no voltage between it and ground.

Warning! You should always turn of the power at the breaker box before working on any wiring. Even though the white wire is usually neutral sometimes it is used as a hot wire such as a traveler from switch to fixture or three-way to three way.
Always check for power with a meter or a led power detector which tells you if there is any voltage in the area.