ETag response header

HTTP 1.1 introduced a new kind of cache validator i.e. ETag.

ETags are unique identifiers that are generated by the server and changed every time the resource is updated. An ETag is a string that uniquely identifies a specific version of a component. The only format constraints are that the string be quoted.

ETag: "10c24bc-4ab-457e1c1f"

The problem with ETags is that they typically are constructed using attributes that make them unique to a specific server hosting a site. So in a typical clustered environment, if the next request for a cached resource goes to a different server, then the ETag won't match and the resource would be downloaded again.

Most modern Web servers will generate both ETag and Last-Modified validators for static content automatically; you won't have to do anything.

More info about ETag can be found at the following links:
http://developer.yahoo.com/performance/rules.html#etags
http://nerdmonkey.com/blog/archives/000098.html
http://www.web-caching.com/mnot_tutorial/how.html

The advantage of ETag over the 'last-modified-date' tag is that the fact that ETags don't require a date but will take any string. So U can compose ETag with any custom logic and source.