What is a 3-way switch?
A 3-way switch is one that allows you to control a ceiling light (or other electrical fixture) from two separate locations. Common scenarios would be 3-way switches located at both the top and bottom of a stairway, or having 3-way switches next to doors in a room with two entry points.
If you look at a 3-way switch you’ll notice it looks different from a standard “single pole” switch. The actual switch is larger because there’s more going on inside. And there’s an extra screw terminal on the side that allows a connecting point for an extra “common” wire so the switches can “talk” to one another. And you’ll notice, the toggle lever won’t have ON and OFF markings. You may even encounter 4-way switches—both in your house and in the electrical aisle of your home center—that allow you to control a fixture from three different locations.
It’s pretty easy to replace a 3-way switch; after you shut off power and withdraw the switch take a quick photo, then move the wires—one by one—from the defective switch to the new one.