The Ken Burns effect adds a dynamic sense of motion to a slide show by panning across photos and zooming in and out.
Two slide show themes allow for Ken Burns moves: Classic and (brace yourself!) Ken Burns. With the Classic theme, no photo has a Ken Burns move, but you can set one up for any photo using the instructions on this page.
With the Ken Burns theme, every photo has an automatic Ken Burns move: iPhoto pans and zooms using settings that it deems appropriate. You can customize those moves, and you can turn Ken Burns off for certain photos if you like.