League of Legends is a popular free-to-play multi-player game developed by RIOT. Despite being free, the game very much has a pay-for polish to it. This is because that while the game is free-to-play, players can pay for micro-transactions that can unlock skins, XP boosts and other things. While the use of micro-transactions is controversial in many environments, the League of Legends community manages it well since everything non-cosmetic can be obtained for free over time anyways. The only thing obtainable with money are skins
League of Legends is part of the MOBA (Multi-player Online Battle Arena) genre. This is generally classified as an action RTS (Real-time Strategy) game. LoL is the most popular spin-off of the original MOBA map/game, Defense of the Ancients, which originated was a Warcraft 3 custom map. While there are other games out there, League of Legends is the most popular.