Flickr is a photo sharing website that allows users to upload and share photos publicly or privately. It has over 5 billion photos uploaded by users from around the world. Flickr also has an API that allows developers to access Flickr data and build applications that interact with Flickr features like photo uploads, tags, and comments. The API documentation provides over 100 public methods that can be used to access Flickr functionality through various authentication methods without using passwords.