User Account Control (UAC) is a Windows security feature that helps prevent unauthorized changes to the system by requiring administrative approval for actions that could affect system security.

How UAC works:

1. Standard user context: Users run with standard user privileges by default
2. Elevation prompt: Administrative actions trigger an elevation prompt
3. Admin approval mode: Even administrators run with standard privileges until elevation
4. Secure desktop: Elevation prompts run on a secure desktop to prevent spoofing

UAC levels:

• Always notify: Prompt for all administrative actions
• Notify when apps try to make changes: Default setting, prompts for unknown applications
• Notify when apps try to make changes (no dimming): Same as above but without secure desktop
• Never notify: UAC disabled (not recommended)

Benefits: Reduces impact of malware, prevents accidental system changes, promotes principle of least privilege.