So what the ******** is this about, then?
Glad you asked. It starts here:

This is Defense of the Ancients. It started off as a user made Warcraft III mod. You can read about it
on Wiki, but here's the jist of it:
You have a dude or wench. So do the other members of your team. You have a base! You don't want it to be destroyed. There is another team. You want to destroy their base.
Bases constantly spawn minions, and send them out in waves. Minions are important, because between the bases are lanes are turrets. Turrets will ruin your Champion's s**t. But if the minions get in a turret's range first, the turret will target them instead, allowing you to smash the turret.
As you kill the enemy's minions, and, hopefully, the enemy champions, you're going to get gold. You can go back to your base and buy items to improve your champion! And, over the game, you'll level up! When you level, your stats go up, and you can increase one of your Champion's 4 skills.
By the end of the game, you'll be level 18, smashing turrets and inhibitors, spawning super minions that are harder to kill.
That's a really long experience, but it's a game with surprising depth.

And so we come to League of Legends. Created by key members of the DotA team, the game is a full-fledged retail game, with 70 Champions and growing. Every week, the Dev team rotates in 10 of the Champions. These are free to everyone for the week! You can earn currency for playing, which you can use to permanently unlock them!
Everything in the game can be earned with in-game currency, with the exception of skins. That said, it is certainly possible to spend real currency on the game. If you're interested in the game and want to spend some cash, I recommend you buy the retail version at Target for 20 dollars. You'll get 20 Champions, and 1380 Riot Points, which are what you get when you spend real money n the game. If you drop another 15 dollars in RP, you'll have enough to buy another Champions Pack from the in-game store, bringing your total to 40, from the getgo.