AppLock allows you to lock access your applications with the password of your choice; you can also force them to quit when the computer goes to sleep so as to be certain that no one will access later !
The password you should enter when you first run the software should be used to unlock the applications, to open the preferences panel and to quit AppLock.
AppLock works in background and is controlled via the menu bar.
Finally, remember AppLock is only compatible with Lion !