High: Vista equivalent
Prompts for: all elevations
Prompts on: secure desktop

Medium: default
Prompts for: non-Windows elevations
Windows means:
Signed by Windows certificate
In secure location
Doesn’t accept control command-line (e.g. cmd.exe)
Prompts on: secure desktop

Low:
Prompts for: non-Windows elevations
Prompts on: standard desktop
Avoids black flash and user can interact with desktop
Possible appcompat issues with 3rd-party accessibility applications

Off: UAC off
No Protected Mode IE
No file system or registry virtualization