Home Server, code-named "Quattro," is all about centralization and ease of use, and it's targeted at users who want an easy way to back up their important data and share media in a home network. The OS likely won't be available to buy on its own though, users can obtain Windows Home Server from resellers who will offer the server software in "OEM bundles" with hard drives, motherboards, or even just cables. Also, a 120-day evaluation version is expected to be available for download. Microsoft has not yet announced pricing.
Microsoft also hopes to get developers interested in building Windows Home Server add-ins, they released a WHS SDK back in April, and have launched a contest offering industry exposure a cash prizes to those who create the best software and hardware add-ins.