The version of Mac OS X 10.5 that will come preinstalled on all new consumer Macs starting this coming Friday is, strictly speaking, the 'client' version of the OS. There's also a server version, which will also be released on the 26th.
The foundation of the two is the same. What differentiates the server version is that it comes with a suite of administration tools and server programs.
Like the client version, it's designed to be easy to use, easy enough that it's a reasonable choice for groups and company's too small to have an IT department, even easy enough for home use for those who need the sophistication of a server.
That's not to say that it's been dumbed down, Mac OS X is Unix, after all, and you have only to open the Terminal program to be reminded of that fact. Any standard tools that may not have been encapsulated in graphical interfaces will still be there, just a command-line entry away, and any other POSIX compliant software available as source code should compile without modification.
Personally, I'll be getting the client version, since I only have one computer, at present, and no real need for the server programs. I wish I could justify the server version, because I'd love to play around with Wiki Server, iCal Server, and iChat Server! Taken together, these server programs and the client programs that work with them comprise some pretty fancy groupware, capable of significantly improving productivity in situations where they're a good fit.
So maybe you're not yet ready for Mac OS X Server either. Just remember that it exists and keep an eye peeled for circumstances in which it might be the right solution. You may find that such circumstances are commonplace, practically everywhere you look!