Although Dota 2 has been playable--competitively, at that--for the past two years now, it's technically been in "beta." Valve promised that its MOBA would officially launch before August, and in a weird reversal of Valve Time, it appears they've launched early.
Grabbing the free-to-play game off Steam will add you to "the launch queue." That means you won't get immediate access to the game. Instead, you'll receive an email "when we are ready for you to come play," Valve explains on their official website. "The volume of players let in will gradually increase so we can spread the launch load out over time."
The launch only applies to the PC version of the game. Valve explains that the Mac and Linux builds are currently in testing and will be available soon.
Once you gain entry into the game, you'll be placed into the New User Experience which has you playing a tutorial and then playing some practice matches against bots. Once you're ready to start playing full games via matchmaking, you'll be matched with other similarly-skilled players. This way, newcomers won't be decimated by the two year veterans, while the experts won't be frustrated playing against n00bs.