An image gallery will display a set of thumbnail images attached to a particular post or page. When clicked, gallery images are displayed in a full-size carousel view. When clicked, gallery images are displayed in a full-size carousel view.
If you upload images using the Add Media button above the post/page editor, they will be attached automatically. If you want to include images that you’ve already uploaded, you’ll need to attach them to a post or page before they will appear in a gallery. http://en.support.wordpress.com/images/gallery/
resource link for galleries > Not the Official WordPress.com Gallery FAQ.
A slideshow will display a set of images attached to a particular post or page. If you upload images using the Add an Image button while editing/creating the post or page, they will be attached automatically. If you upload images directly to the media library from Media -> Add New, you will need to attach them to a post or page before they will appear in a slideshow. http://en.support.wordpress.com/slideshows/