When Windows launched Vista in 2008, it brought a large list of improvements over previous versions. One of the largest selling points for Vista was the increased security that it offered and one of the central aspects of this increased security is UAC, but exactly what is UAC? Windows Vista has the built-in ability to automatically reduce the potential of security breeches in the system. It does that by automatically enabling a feature called User Account Control (UAC). The UAC forces users that are part of the local administrators group to run like they were regular users with no administrative privileges.
Whenever a user that is a member of the local administrators group (or even a member of the Domain Admins group if the computer is part of an Active Directory domain) tries to perform a task that requires administrative privileges, the operating system halts the operation and prompts the user to acknowledge it prior to running the task.
Note the little shield icon next to some of the items in the above screenshot. These items, if clicked upon, will invoke the UAC prompt, and the following message is displayed:
In case the user is not a member of the local administrators group and he or she tries to perform a task that requires such privileges, they are prompted to enter the valid credentials of an administrator (similar to the Run As command in existing Windows XP/2003):
Although UAC clearly improves the security on Windows Vista, under some scenarios you might want to disable it, for example when giving demos in front of an audience (demos that are not security related, for example). Some home users might be tempted to disable UAC because of the additional mouse clicking it brings into their system, however I urge them not to immediately do so, and try to get used to it instead.
In order to disable UAC please read the following article: Disable User Account Control in Windows Vista.
In this article I went over some of the history behind UAC and how it works. Hopefully this brief articles makes it clear what UAC is.