Minecraft: Redstone Loops
So what is a redstone loop...
A series of Redstone Repeaters placed so they direct the signal flow in a endless circle.
The smallest loop uses 4 repeaters. (and is very difficult to start)
How to start a loop?
Use a lever and quickly turn it on then off to send a single signal... and that's it.
Now how do you stop a loop?
Simply break the loop either redstone or a repeater and just wait for the signal to essentially "die". (replace it back and you can restart the loop again)
A more advanced version would be use a piston and lever with say cobblestone to "lock" or "unlock" the signal's flow. (starting is the same but stoping doesn't require you to break then replace what you broke, this is also more expensive)