Flickr is a photo sharing website, that allows users to upload photos and place tags on them. This means any photo you upload will be open to EVERYONE including people you don't know and don't want to know.
Flickr does allow your photos to linked directly to other pages with a STATIC id, a very useful feature for posting to forums.
In addition FLICKR keeps track of data such as the camera you used, the lens, the F-stops, Etc, and then allows people to view your photos even without any tags because because you used a popular camera.
While sharing photos is a popular thing, i prefer to share my photography with people I know
photos.yahoo.com allows you to make ONLY people you invite see your photos. This was a much better service until
1) Yahoo upgraded it making it much more WEB 2.0 and harder to use
2) Yahoo decided to disban in this fall so everyone was forced to you FLICKR.
In addition unlike photos.yahoo.com Flickr has upload and download limits.