It's 99% Free to Play. The only thing that's limited to the purchase currency is skins for your Champions. But all the Champions are achievable with the in-game currency.
You control a Champion of your choice. You can team up with other people against bots, or play a 5v5/3v3 match with all real players.
Atm, there's only two maps, a 5v5 map & a 3v3 map. You have in-game level's and, in a way, "real" level's outside of the match. The max lvl is 30. Each time you level up, you get a Rune Slot & a Mastery point. (Don't buy Runes till you're lvl 20). Masteries boosts Armor/Dodge/Magic Resistance/Crit Chance/Damage etc. And you can refund the points depending on if you want to play as a Tank Champion, Assassin, Mage etc. And also, in the earlier levels, you unlock Summoner Spells. (Two spells you can take in-game, Heal, An escape ability, Teleport etc)
When you start a match, you always start at lvl 1. And while playing you can go up to lvl 18. And each time you level up in-game, you can choose a spell for your Champion. All Champions have 4 unique spells. One of them being an Ultimate Spell, that you always unlock at lvl 6. When you kill champions or minions (Neutral Creeps) you get gold. You can use gold to buy items that can give you bonuses such as movement speed, lifesteal, attack speed, attack damage, ability power and etc.
I find the game very balanced. The dev's listen alot to the community. It's a matchmaking system, but it works alot better than games like BFH.
League of Legends has around 75 champions if i ain't mistaken, with a new one every 2 weeks or so, and all are buyable with in game currency. You also have a free Champion rotation whch changes every week, to test new champs / play with champs you havent bought yet.
Champions go from horrible monsters to midget inventors, ninjas, a vampire, a living tree and much more
So if you feel interested and want to play - Clicky ^^
There are NA servers & EU servers. I play on the EU servers.