Server software as an add on downloadable from the App Store on the Mac, not a physical server like XServe. They recommend a Mac Mini with two hard disks installed as a SOHO server but obviously aren't catering for large neworks - no hot-swappable drives or fancy RAID for example (unless you use an external Thunderbolt caddy). They're leaving that market to Windows AD and Linux (mostly Windows AD I suspect). Oddly, the server software on the Mac gets less and less features each release since Snow Leopard apparently. I think Ars Technica has a review of the server software, and is pretty in-depth.