Flickr is a photo sharing website that allows users to upload their photos and share them with others. To use Flickr, a user must first create a Yahoo email account and then sign up for a Flickr account on their website. Once logged in, the user can upload photos by clicking the "Upload photos" button, selecting photos from their computer, and waiting for the photos to finish uploading. After uploading, the user can add details like names and events to their photos, which are then visible online for others to see.