Galleries are a list of images, and can be based on the contents of a directory, flickr tags/users or tumblr logs.
The module tries to make creating galleries fast and easy, and user experience more seamless using AJAX for displaying images, but is fully usable without javascript and/or without CSS. A basic example website with Drupal's default settings and a few galleries can be visited here: http://rivag.es - keep in mind that it could look much better with a better adapted theme.