Valve announced that Dota 2 will be free free-to-play and revealed details on the Dota Store today. The game will feature an in-game store that will offer mostly cosmetic items, just like the offerings provided to Team Fortress 2 players. Valve says that it has no plans to sell hero characters in the Dota Store because "restricting player access to heroes could be destructive to game design, so it’s something we plan to avoid."
Valve also revealed that Dota 2 will be part of the Steam Workshop, the company's service that lets fans share content they’ve created for games. The system allows fans to vote on content, with the most well received bits being released by Valve as official parts of the game. Sales of the content are then shared with content creators. Valve claims that it paid out over $3.5 million dollars to Team Fortress 2 fans whose creations became a permanent part of the game in the first year it became a free-to-play title and utilized Steam Workshop.
You can learn a whole lot more about the Dota 2 Store in this official blog post.