User accounts on OSX Server

Like Active Directory, except it's a heck of a lot easier to set up and manage users, groups and machines.
A LOT easier.

http://www.apple.com/server/macosx/technology/open-directory.html

 

Yes, in fact you have a couple of options. You can set up "network accounts," where the users' data reside in network folders...when a user logs on to a new machine, the Home Folder is set to the network share. This is good for desktop machines that are always connected to your network. Another option is to setup "mobile accounts," which synchronize the users' data between the server and the client. So, when a user logs on to a machine for the first time, his data are synced to the client, and any changes are synced back to the server. This works pretty well overall, but is sometimes a bit flakey. This scenario is ideal when you have laptops that might not be connected to the network at all times.

You can read up on OS X Server's full range of capabilities in Apple's excellent Server Guides. Good luck!