Gates are circuitry that function as signal transfer stations. Basically, each redstone torch placed inverts the redstone signal, so you have:
-------oxxxxxxxxxxo---------
Where o is the torch, - representing the on signal, and x representing the off signal.
It's a way to send redstone signals further distances without having to go all the way down to one end to flip a switch.
It's a useful way to power multiple sections of powered rails at once (
in my case), or other devices without needing to flip many switches that may be in hard-to-reach places.
Also, some gates are able to act as timers by following specific guidelines to place the wiring and torches in a way that they function as clocks.
Some examples of redstone Logic Gates:
http://www.minecraftwiki.net/images/c/cf/StandardLogicGates.pngAlso, if you've seen some of those large dispenser walls (
like this one:
http://www.youtube.com/watch?v=ewFrgDPCgGA), they are powered by redstone logic gates as well.
