LED tellies are actually LCD tellies - the only difference being that they use LEDs for the back-light instead of fluorescent lights.
Manufacturers claim that this can result in brighter picture with enhanced contrast. Personally, they all look the same to me. An LED telly will run slightly cooler, be slightly lighter and use slightly less power.
It also means that the case can be made even thinner so that they often have even worse (if that's possible) sound quality than standard LCD tellies.