ETag is part of http the protocol for the world wide web it is one of several mechanisms that http provides for web cache validation and which allows a client to.

An entity tag (ETag) is an HTTP header used for Web cache validation and conditional requests from browsers for resources. Etags use persistent identification elements (PIE) that have been tagged to the user's browser.

ETags are used to check if the browser cache is valid or not. Apache creates ETags using file size, inode and the last modified date.Etag is useful mostly for static contents

The ETag or entity tag is part of HTTP, the protocol for the World Wide Web. It is one of several mechanisms that HTTP provides for web cache validation, which allows a client to make conditional requests.