Like DOTA and HoN, the essence of the game is basically to destroy the opposing team's base. The teams consists of 3 or 5 players, and each player picks a champion (there are 72 in total, and growing) before the game. Every champion is unique and contributes to the team in their own way, giving you an endless amount of possibilites when setting up your team. The thing that makes LoL really stand apart from similar games is the Influence Points-feature (IP). IP is LoLs ingame currency and is earned after every game, regardless of outcome (though you of course get more if you win). This is used to buy champions and Runes, and give you a more close relationship with your champions (pardon my freaky-ness), than in DOTA or HoN were the characters are all avaliable at all times.
Really don't feel like going in to more detail at this point, as you can get a much better overview at their webpage. But I'll certainly make more posts in the future for worshipping this game. Yeah, and I forgot... IT'S FREE TO PLAY!
Here's their own cinematic trailer, which is just brilliant
Forgive me for what seems to be an unfinished posts, but I'll be back