Valve has introduced the ability for developers and publishers to offer downloadable content through Steam, just as Microsoft and Sony do with their console platforms.

This means that any game attached to Steam - whether it was bought direct, at retail or elsewhere online - can now use Valve's platform as a one-stop shop for add-ons.

It also means that publishers are now in a position to charge for downloadable content through Steam.

Source: Gamesindustry.biz