The "app store," or "Windows Phone Marketplace," was technically introduced eight months ago as a place where users could download software and updates for Windows Mobile 6. With the advent of Windows Phone 7, however, there will be a new set of policies governing the Marketplace, according to a blog post by Brandon Watson.
Notably, developers will have to shell out $99 per year (the same as for Apple's App Store) in order to submit applications. For the $99 fee, developers will be able to submit five free applications (additional free applications will cost $19.99 each--hopefully this will cut down on the spammy/stupid apps) and unlimited paid apps. The paid apps feature a 70/30 revenue split, which is the same deal Apple gives its developers.
Windows Phone Marketplace also offers a slew of options not available to Apple developers--most notably, a "Trial API" option, in which users can try out apps before purchasing them. The length of the trial period and type of trial is controlled by the developer.