I found this by accident when I was installing SQL tools on my Vista machine. I think it gives a pretty good short explanation of User Account Control.
http://www.microsoft.com/sql/howtobuy/windowsvistasupport.mspx
This paragraph starts the helpful explanation:
Microsoft Windows Vista includes a new feature, User Account Control (UAC), which helps administrators manage their use of elevated privileges. When running on Microsoft Windows Vista, administrators do not use their administrative privileges by default. They perform most actions as standard (non-administrative) users, temporarily assuming their administrative privileges only when necessary. This separation of privileges causes some known issues.