The best Free to Play Game on the Interwebs now.
You, as a Summoner, play from an overhead third person view as a Champion, dealing death to the enemy (Blue or Purple) minions, and waging epic (or not-so-epic) battles against enemy Champions. While playing, you earn gold, use it to buy items in your base's Shop; Level up to 18 (start at level 1, get to 18, finish that game, start another game and begin on your champion at level 1..), and sneak through bushes and destroy Turrets on the lanes, raze enemy Inhibitors for super minions, and ultimately work to demolish the enemy Nexus.
While playing, you can also traverse through the 'Jungle' area, killing neutral (and epic) monsters for buffs, extra gold and experience.
Over 50 Champions to pick from; Champions are rotated for 'Free Play' each week. Purchasable through IP (Influence Points), which are obtained via playing games. Alongside those are Mastery Points (1 per "Summoner Level"), and which are used for Mastery Talent Trees (WoW-vets will know how to make this out).
Alternate Champion Skins are purchasable through Riot Points, and Runes for your Rune Pages are bought with IP. The game itself is an average of about an hour to two hour download time, and there might be patches to download and install.
Two maps to play on: Summoners' Rift (5v5), featuring three lanes; Twisted Treeline (3v3), with two lanes.
Discuss the Epic-ness that is LoL.