They are called three-way-switches. They are both wired into the same circuit that controls the light.
It is dependant on whether the switches are in wired in a parallel or series configuration. If the switches are wires in parallel then both switches would have to be off to turn the light bulb off. Either switch could turn the light bulb on. If the switches are wired in series then both switches would have to be on to turn the light bulb on. Either switch could turn the light bulb off.
When there's two switches connected to the same lamp it's usually because the lamp was meant to be able to be turned on and off from two different places. Useful in corridors and stairs and places like that. Or it can be that it originally was a fixture with more than one bulb in it. Two switches meant you could get different amounts of light out of the same fixture.
The use of a four way switch is needed when a load (such as a light) needs to be controlled from 3 locations or more. For instance when you walk down a hallway and there is a light there are probably at least two switches one at the beginning of the hallway and one at the end. These two switches will turn the light on or off from either location. These two switches will be three way switches. When the need to control a light or a load from more that two locations arises a four way switch is introduced. A light or load can be controlled an unlimited amount of locations by adding four way switches which are wired in between the two three way switches. Think of a sandwich where the three way switches are the bread and the meat and veggies are the four way switches. You can put as much meat and veggies as you want, but you always need two slices of bread on the top and bottom.
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