What's a global catalog server? It's a domain controller w/ searchable information about Active Directory objects stored in other domains throughout a forest. I think that the first domain controller in a forest is automatically selected to be a GC. In a single-domain environment, you only need that first GC. In multi-site, multi-domain environments, you may need to appoint other DCs as GCs.
Microsoft's article gives a quick run-down on how to enable/disable a GC, a plain-english article on computerperformance.co.uk summarizes the role of a GC, and a particularly helpful TechNet article discusses GCs. If you logon w/ a UPN (email address), your workstation contacts the GC for a DC to authenticate to.
