Flickr asks photo submitters to organize images using tags (a form of metadata), which allow searchers to find images related to particular topics, such as place names or subject matter. Flickr was also an early website to implement tag clouds, which provide access to images tagged with the most popular keywords.
Flickr also allows users to organize their photos into "sets", or groups of photos that fall under the same heading. However, sets are more flexible than the traditional folder-based method of organizing files, as one photo can belong to one set, many sets, or none at all. Flickr's "sets", then, represent a form of categorical metadata rather than a physical hierarchy. Sets may be grouped into "collections", and collections further grouped into higher-order collections.
Finally, Flickr offers a fairly comprehensive web-service API that allows programmers to create applications that can perform almost any function a user on the Flickr site can do. To use the Flickr API, users need to know their Flickr NSIDs. Several sites have been developed to aid users in this, including idGettr and What Is My Flickr Id.