Like iPhoto 5, iPhoto 6 offers two types of slideshows, which I call "basic" slideshows and "saved" slideshows.
About basic slideshows:
Basic slideshows are akin to the slideshow feature in earlier versions of iPhoto. They're simple to use and offer a basic set of options that apply equally to all slides.
Use basic slideshows when you want to show someone a set of photos quickly, without any fuss or bother. Basic slideshows can also be useful for reviewing just-taken photos and culling the lousy shots.
Lastly, you can use only basic slideshows when viewing images from a shared iPhoto Library over the network.
About saved slideshows:
Saved slideshows appear in the Source pane like albums, books, cards, and calendars, and any changes you make to them are saved for the future. You can organize saved slideshows in folders, duplicate them to experiment with different approaches, and export them to QuickTime movies.
Use saved slideshows when you want to put some effort into making a slideshow as visually impressive as possible. You can add and remove individual photos from the slideshow, apply temporary effects to photos during the slideshow, change the time each slide appears on screen, adjust the Ken Burns Effect for each slide, set the transition between any two slides, and more.
What's particularly neat about saved slideshows is that they're created with default settings, so you can customize them as much or as little as you like.