The core OS is the same as the client version. OS X Server has several bundled server applications. Most of these (including, but not limited to Postfix, SpamAssassin, Samba, Tomcat, Apache) are either included with the client version of OS X, or are Free/Open Source downloads.
OS X Server, however, adds GUI frontends to these tools that allow for configuration without editing config files (though you can still do that.) Apple also combines several packages, like LDAP + Kerberos +...= OpenDirectory (replaces ActiveDirectory or eDirectory, but based on open source.) Then there are the special Apple server packages like WebObjects and QuickTime Streaming Server.
Basically, it's every kind of server-side technology Apple could think of, with added Mac-style GUI administration tools.