See the table at
http://www.microsoft.com/windows/windows-vista/compare-editions/default.aspx
Enterprise (not listed in the above link) is essentially Ultimate minus
products that aren't generally appropriate for an enterprise environment:
movie maker, parential controls, etc. There are differences between Business
and Ultimate/Enterprise such as the absence of BitLocker in Business.
Business and higher versions can be joined to a domain.
If you're a volume customer you're probably licensed for Business, Ultimate,
and Enterprise, at no difference in cost. Business and Enterprise both can
be volume-activated (either by MAK or KMS), but Ultimate allows only the
retail one-install-per-key although you can get keys as needed from the VAR
through which you purchased your volume license.
Your volume license contains the conditions under which you can use the
Windows systems it authorizes. You need to contact whoever is your VL
administrator to get the gory details (ask for the PUR: the Product Use
Rights document), or contact your VAR if you can't figure out what the
details mean. (I've been in that last boat, having a question that stumped
the VAR and the Microsoft sales team assigned to my account - the request
for clarification went all the way to Microsoft's contract office.)
Joe Morris