Use cases describe the functionality of a system from the point of view of users of the system.
Actors describe the unique kinds of roles that users play when interacting with the system.
Here's how I described it to a friend. The developer cuts tables into 10 pieces so that they depend on loops and stuff. After a while when there is empirical evidence that the design was too artistic, the HTML is messed up by geeky DTML or ASP or PHP code that nobody dares to touch. The designer changes things with the template given from the application developer in Dreamweaver and can during his development hit F12 to view it in an external browser. After a while when there is empirical evidence that the design was too artistic, the boss can ask the designer to redesign the template.