flickr is a Yahoo! photo hosting and sharing service. I started using it in 2009 because, unlike facebook, it allowed me to keep the rights to my photos and alter them if, and when, I wished.
Originally I found it rather frustrating with upload limits and being unfamiliar with the process. Two things happened to change that. For Christmas my wife got me a pro account, which meant I had unlimited uploads and I downloaded the Uploadr extension. Flickr Uploadr is a downloadable program that easily allows for mass photo uploads. As well, it allows you to pre-organize your photos. I can put pictures in a set or create a new one, change the title, add a description, tag and determine the level of privacy. All this before I even upload them.
With a pro account and Uploader I have not looked back. It has been a great way to share our travels with family and friends. (Especially for ones not one facebook as I leave the vast majority of my pictures free for public viewing.) It also has been fun getting feedback from the public on my pictures. I've even had interest in some of my pictures for publications.
Photos can be organize in both sets and collections. Sets are akin to albums, whereas collections are thematic collections of albums. When we travel I create a collection for the entire trip and use sets for specific days or places.
My camera doesn't have a GPS, so geotagging is a labour intensive process involving remembering where I was when I took the picture and finding it on a map after uploading it. However, it was great to increase the depth of sharing our travels. People see what we're doing and also where too.
23, Empty Seats, and STOP! Signs (Ode to the Octogon) to name a few. I created and oversee Winnipeg Expats, a group dedicated to memories of Winnipeg.
Sharing photos with groups allows your pictures to reach a wider audience and lets you see what other people with the same interests are taking pictures of.
Educationally, groups can help guide students. Doing a research project on the history of particle physics? There are particle physics groups on flickr. Want to show students what types of flowers grow in Australia? Search "flowers" taken in "Australia" on the flickr world map and there you go.
The social aspect of flickr, contacts, comments and such, is another thing that makes it great. I have contacts that I know personally and contacts that I will never meet. Their feedback has been very helpful in my growth as a photographer, as has been viewing their work.
I think flickr users are very proud of the social nature of the site and thus many people are committed to making their photos available for everyone. Flickr gives the option to add photos to the creative commons, with specific licencing options. The licencing is straight forward and easy to see on every picture. As the majority of pictures I share are of personal travels I keep my photos "all rights reserved", but share if people ask.
The sheer quantity and variety of photos available, plus added benefit of a huge selection of free to use photos, makes flickr a great tool to use in education.