Flickr has an open API. This means that anyone can write their own program to present public Flickr data (like photos, video, tags, profiles or groups) in new and different ways. The App Garden is a place where developers can showcase the applications they've created and where you can find new ways to explore Flickr.

With over 5 billion photos (many with valuable metadata such as tags, geolocation, and Exif data), the Flickr community creates wonderfully rich data. The Flickr API is how you can access that data. In fact, almost all the functionality that runs flickr.com is available through the API. And the API is completely free to use, as a service to our members as well as developers and other integrators, so they can create even more ways to interact with photos beyond flickr.com.

Check the Terms of Use here