If your computer is describing what's on your screen, or if a black box is following your actions with the mouse and keyboard, VoiceOver may be turned on. VoiceOver is an alternate Mac OS X interface for people with visual disabilities and others who need to hear what's on the screen.
To turn off VoiceOver:
Press Command-F5. (On a portable computer, you may need to press Command-Fn-F5.)
If sound is turned on, you hear the computer say "VoiceOver Off" to confirm your action.
Note: If you turn off VoiceOver but it turns on again the next time you log in to your computer, open Accounts preferences, click Login Options, and deselect "Use VoiceOver at login window."