Here’s the description from the Astoria Project Site (where you can get the CTP today): http://astoria.mslivelabs.com/.
“The goal of Microsoft Codename Astoria is to enable applications to expose data as a data service that can be consumed by web clients within a corporate network and across the Internet. The data service is reachable over HTTP, and URIs are used to identify the various pieces of information available through the service. Interactions with the data service happens in terms of HTTP verbs such as GET, POST, PUT and DELETE, and the data exchanged in those interactions is represented in simple formats such as XML and JSON. “
Here’s a great video by Pablo Castora: http://channel9.msdn.com/ShowPost.aspx?PostID=306736#306736