The original Defense of the Ancients, better known as DotA, was a Warcraft 3 mod that saw an immense following from around the world. However, due to the WC3's engine limitations, it was only inevitable that companies would remake the game with their own technology. Though certainly not the first title to be based off the original DotA, Valve's Dota 2 has one thing it can boast over other MOBA/ARTS games: They have two key members of the original DotA team onboard. Valve has ensured that Dota 2 will not be pay2win and that items will only be cosmetic, which has already been evidenced by the numerous hats in the ingame shop (not that it's a bad thing). All heroes will be readily available to all players, with only a few of them needing to be ported from the original DotA left. The current roster however surely holds enough variety for everyone's own tastes; whether you want to play as a high DPS mage slayer who hates magic with a passion, an insane pyromaniac who rides a bat and specializes in crowd control, or even a giant scorpion that creates earthquakes, surely Dota 2 will have a hero you will like.
Some handy links. I really recommend reading/watching these. Don't be too intimidated by the vast amount of info, everyone has to start somewhere and I'll be glad to explain/play with everyone that needs help :v