Electronic relay, not running
Think of a relay as an electrically operated switch. An electromagnet closes a switch. When the electromagnet is powered, the switch closes and turns something on. When the electromagnet is not powered, the switch opens and turns something off. Relays are usually used to turn higher-voltage circuits on and off. For example, if I want my microcontroller (which runs on 5v DC) to switch on my lights (which run on 120v AC), I can use a relay. The microcontroller powers the coil, which closes the switch and turns on the light. Another added benefit is there is no common point between the low-voltage circuit and the high-voltage circuit, so there's not much chance of harming the microcontroller.
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can u post a diagram of a relay please
So, it's pretty much a self-controlled Reed Switch?
Yes, it can be thought of in that way. The reed switch is the switch portion of the relay. The other half would be some kind of magnet you can control, either a permanent magnet you can move or an electromagnet, that can open and close the reed switch.
Most relays are "momentary": while there is current in the coil, the magnetically influenced contacts stay open or closed as required. It should be noted that the holding current to maintain this switched state can be significantly less than that required to trigger the swtich. There are also "flip-flop" relays which use a pulse in either direction to change the switch, but require no further current to maintain their position, a reverse pulse puts them back to the initial position. The Reed swtich is a good, easily understood example.
It's a mechanical switch run by an electrical control. An electromagnet moves a chunk of metal that triggers something.
Or it's a signal repeater-takes a signal weakened by distance and repeats it so that the signal can be read farther away, mostly a telegraph-era use, I think.