What is an ETag?

An ETag is a HTTP response header (not generally seen by the user) that is unique for each URL. Etags are generally used for caching.

If a crawler or browser checks the ETag when the page is first requested and then re-checks it when re-requesting the page, then it knows the page hasn't changed and therefore it can use the local, cached version which saves downloading the document again.

They are generated by the web server for each document. If the document changes then a new ETag will be generated automatically.

Live.com use ETags but Yahoo! do not. It is thought that Google also ignores them but this hasn't been confirmed.