Just found this game a while back and I have nothing but good things to say about it so far. From what I know this started out as a mod built on the Warcraft III engine. The mod was called DOTA Allstars I believe. Somewhere along the line the game became standalone and falls into the category of what I see called Free 2 Play games. Basically this means you can play the game for free or you can pay for in game currency called Riot points so that you can get better Champions or Skins for them to give them a better look and stand out a bit more. If you don't want to pay you can get your Champs the hard way by earning points that you can spend just like Riot points. I have found that there are some things that you can only buy with Riot points, but your experience should be just a good should you decide not to buy them.
As far as game play it is very similar to Warcraft III, the advantage being that you only have to concern yourself with controlling your champion and not all of the AI or "minions". They spawn every so often and mainly do their own thing. You play on teams of 5 and your goal is basically to work your way into and destroy the opposing teams base while making your way past a turret defense on the way.
Give the game a try if you are looking for something new or happen to be on a budget as so many of us are nowadays. Best of luck to you and good gaming.