There are many. In short, a relay is just an electrically activated switch that in many ways is just a like a light switch in your home but with several important differences. Because it is, at it’s core, just a switch, you can use a relay for anything that you would want to turn on an off, but it’s those differences that I mentioned that make things interesting.
This first example I learned the first time I stepped into a lab in college and we flipped on the lights. it was a large room with many banks of overhead lights, but just one ordinary switch by each door. When you flipped the switch, you would hear a solid “thunk” sound somewhere overhead, and then a moment later the lights would begin to come on. What was happening was that so many lights in such a large room drew so much current, that an ordinary switch could never handle it all. And so you could a) use a lot of switched and divide up the load, b) use one really big industrial switch that would be difficult, expensive, and unweildy, or c) use an ordinary light switch, to power a relay that could easily handle such a load. The “thunk” you heard when you flipped the switch was the sound of the relay throwing somewhere above the ceiling.
As I mentioned, since relays are electrically activated switches you can use them for a lot of things but one of the things that makes them different is that, depending on their design, the voltage needed to throw the switch doesn’t have to be the same as the voltage being switched. For example, if you those lights in the lab are 220 volt lights, you can still switch them with 120 volts or even 12 volts. Depending on the specifications of the relay that means that you could use a relatively low voltage, low amperage output from a computer to control a large bank of lights, or a large motor, or most anything else.
Relays can do all sorts of other things as well. Some have timers built in to them so that when they are switched on (or off) they “wait” for a specified amount of time before switching. Others have a sort of basic logic circuit included in their design so that two or more circuits have to be simultaneously live or “on,” before they switch, meaning that you and essentially use the relay to say “IF this AND that, THEN switch.” This is the sort of thing sometimes built into safety equipment so that the elevator won’t start until the door is closed (yes, I know that elevators use Programmable Logic Controllers or more advanced logic devices, but you get the idea) or an industrial machine won’t start until the operator pressed two buttons with two hands to insure that his or her hands are not inside the machine.
These are just a few examples. If you spend even a little time looking at a catalog (on paper or online) you will quickly discover that there are more types of relays than you ever imagined and they can do things you probably never thought about. They can, and do, a great many things. So many in fact, that you are surrounded by them every day. They are found in lighting applications, and safety applications like elevators and traffic lights, small relays run the windshield wipers in your car, and really big ones bring the electricity from the power plant to your home.
In the end, relays do so many things, in so many applications, in so many places, for so many people , that we’d really be lost without them.