AD Empty Sites
Active Directory sites are considered empty if they do not contain a domain controller. This immediately begs the question: why would anyone want an empty site? The answer is that there are more applications than just Active Directory that are site aware. This, albeit short list, includes DFS, SMS 2003, SCCM 2007, and Exchange 2007. Thus, just because a site does not have a domain controller, does not mean that the boundary defined by the site is not important for another application.
So just how do clients determine which site that they are in? Sites are of course are defined by IP subnets and at the time a client is joined to the domain, it performs a lookup in Active Directory to match its IP address to a site which is then stored in the registry. This information is updated every time a client logins into the network.
Why do some consider empty sites bad? This stems from the mis-perception that clients in an empty site will not be able to choose an optimal domain controller. Clients find domain controllers by looking them up in DNS based upon the site that they are contained in. Active Directory creates DNS records in empty sites for domain controllers according to the topology generated by the KDC. This can also be manually controlled via Group Policy or the registry on specific domain controllers: How to optimize the location of a domain controller or global catalog that resides outside of a client's site.
Empty sites do need a little extra thought, but they do have a purpose and should not be discarded. For detailed information about how clients find domain controllers, see the Locating Active Directory Servers from the Windows 2000 Resource Kit.