The gallery post format will add a slideshow to the top of that page/post (like this page in the demo http://www.organizedthemes.com/try/blog/2011/10/31/gallery-post-format/).
The original version of this had a small selector on the add image lightbox that let you choose which images were included. I’ve since updated the theme to automatically include all images on that page/post. The original version had a bug that pulled images site wide that were selected and not just on that page/post.
If you’d like to use a thumbnail image gallery (like you are on that page), you don’t have to select the gallery post format. Just add a WordPress gallery like you normally would.
WordPress dictates the names of the post formats. If I were given a choice, I’d use slideshow instead of gallery, but it is the closest I have available.