If you played Defense of the Ancients (DoTA) before, then you'll get the drift of it.
The most common matches are 5v5 where you versus real time players or AI bots. Includes many other variations of battle. It's a game where you choose a champion and use its skills and in-game strategies to defeat your opponent. In-game strategies such as map prediction, accuracy, reaction speed, good timing and placement will all account into a victory.
In fact, most players from DoTA migrated over to League of Legends because League of Legends offer a more standard balanced amount of champions, that none of it gets too OP (overpowered) half way into the game like DoTA does and just totally wrecks the game. On the other hand, in DoTA for example, if you have OP heroes like troll warlord and you got fed, you would just insta-win the game. But in league of legends, there's no such thing as "OMG IM FED I WIN" mentality. I've personally went on a 5v5 ashe with 49-7-15 and still lost the game, because you see, it's not about being OP, it's all about teamwork, it's all about cooperation.
Nonetheless, I recommend you give it a shot, recommend it to your friends as well!