Finally, Influence Points also let you purchase new Champions, which is League of Legends' name for Heroes. A shortcut to Influence Points is to simply buy Riot Points with real money, which explains where the developers intend to get at least part of their income, but it's honestly not necessary.
In general the Summoner System is a really smart addition. It is, in fact, so smart that it's a little fearsome that Riot Games is still putting League of Legends out for free. After being passed from talented modder to talented modder and enjoying iterative design for six years, polishing Defence of the Ancients was always going to be like trying to sharpen a cut diamond. The Summoner System has let Riot Games build on DOTA without upsetting its patented secret formula.
Not that League of Legends doesn't alter the game. It does, but quietly. Shh! Teamplay is encouraged by things like assisted kills earning you more XP, action is given a shot in the arm by having a lower average ability cooldown time, and slower, more tedious defensive play is hampered by removing the ability to kill your side's minions (therefore denying the other team the XP). League of Legends also adds Brush, which is tall, grassy terrain that blocks line of sight and opens up new avenues for cunning play.
Finally there's a host of practical and cosmetic changes. There's a great automated matchmaking system for dropping into games, an improved mini-map and interface, fantastic audio in both the game's music and the voice acting for each Champion, and a lovely Warcraft-inspired yet slightly cel-shaded art style.
But maybe you never played Defence of the Ancients. Maybe you just want to know, right now, what this game is like to play. And I will tell you this: It is like riding a toboggan down a hill covered in not snow, but numbers. It is joy. And it's infinitely more tactical and tense than you would think ordering a single unit around could ever be, because it's basically a race.
Any enemy champion who's three or four levels above you will have very little trouble grinding you into the dirt, and if it's a particularly aggressive Champion (there are currently 38 to choose from and they all have wildly different skills) you might not even be able to run away. So while actually winning the game is a matter of joining a charge of minions and knocking down the towers that lead to the enemy base, you're always hunting for easy experience.
The thing is, every time you die it takes you an increasingly long time to respawn, soon reaching the agonising heights of 60+ seconds where you can do nothing but think of the XP you just gifted your killer, and when you're dead you can't be gaining experience. Born out of this is an utterly brilliant risk-reward mechanic. Do you go limping around the map with half health and keep fighting, knowing you're an easy target, or do you return to base where you can quickly regenerate your health and mana? Enemy heroes lurk around every corner, and in LOL they're in every patch of grass, too.