Apple has released a developer's preview of OS X 10.7 (Lion), which incorporates OS X Server within the standard OS X client. Until Lion, OS X client and OS X server were two distinct software products, with different licenses and a different customer base. OS X Server was used primarily in small businesses, labs, education, and workgroups within larger corporations. The OS X client was, well, used by everyone with a Mac.
Lion appears to meld the two together. That may sound like a difficult process, but OS X client and OS X server weren't all that much different. In fact, if you didn't mind poking around under the hood, you could use OS X client as a server; you just had to manually configure the various services on your own, using Terminal, configuring files, and sometimes building needed services from source distributions. OS X server put a simpler user interface over the complex process of creating and maintaining a server environment.
With Lion, Apple appears to be giving the end user the choice. You can run Lion as the standard client, or with the flick of a software switch, run Lion as a server. What's not clear is whether Lion Server will use the same license as the client, or if turning on the server features will require a trip to the Mac App Store.
One of the new 'server' features is File Sharing for iPad. With this service and your iPad, you'll be able to access, copy, and share documents you store on your Lion server. I can see those of us with iPads and Macs in the office or home, using this server feature to work seamlessly on documents. We'll be able to sit at the office with the Mac using Pages to work on documents, and then pick up the iPad in the evening to review and make final edits before sending documents off to their final destination. Of course, you can do this now if you wish to use a cloud-based system like MobileMe, but with the Lion-based File Sharing for iPad, you can do so in a self-contained environment within your home or office, without the need for any type of public access.