A “carousel” is a slideshow of images which can be set up as a self-running slideshow or triggered by tabs or buttons. Rather than a fading slideshow, each image will slide into view (either from the side or from the top or bottom) then, when the last image is reached, the carousel will slide back through the entire slideshow to start again from the beginning in a constant loop.
The Carousel Action can be downloaded from ActionsForge at http://actionsforge.com/actions/view/15-carousel and the best learning resource for how to set it up is the free Graphic Carousel screencast which you can find at http://danjasker.com/store/screencasts/free/.
A sample carousel (which is one triggered by tabs and buttons) can be seen at http://scripty.walterdavisstudio.com/carousel/#instructions.