League Of Legends is a game with a long evolutionary history. If you're already familiar with it, feel free to skip ahead a bit, but everyone else will need some explanation as to how this particular free online game came to be.
First, Blizzard made Starcraft, then someone whose identity has been lost in the mists of time made a custom map for Starcraft called Aeon Of Strife. While it was made using the nuts and bolts of Starcraft, this map required a very different style of play a regular Starcraft map and it spawned many imitators, to the point that Aeon Of Strife (or AoS for short) became not just a map, but a map type. And not just a map type in StarCraft, modders of Blizzard's other popular RTS Warcraft III also got hold of the idea with one variant in particular emerging to become one of the most popular mods in any game ever.