ETag(Entity Tag) is the one which is used for to saving the bandwidth and computation cycles with the help of HTTP Caching. It Improve the performance of dynamically generated pages.

The HTTP Protocol specification says ETag as the "Entity value for the requested variant" (Refer:- HTTP/1.1: Header Field Definitions - Section 14.19.). Another way of specifying ETag is the token that can be associated with web resource(Normally, a Web Page). It represents the HTTP Response header returned by an HTTP1.1/Compliant web server used to uniquely identify the version of a page.

ETags are used in conjunction with "If-None-Match" header on GET Request by the savvy server developers to take advantage of the clients browser. Because, the server is the one generating the ETag in first place, and then later it used for the purpose to determine whether the page has changed or not. If it not means, it responded with Not Modified response header(Code:- 304). However, the client request the server to validate the ETag for to acheive this.So, by this way we can save the bandwidth and computation cycle

1. First the Client send a Request for the Page (X)
2. server sends the page (X) back to client along with the ETag
3. Client send a request for the same page(X) again with ETag got back from server
4. Server validates the ETag and determines the page hasn't changed since last time the client requested it. If so, server sends the 304 Response with empty body.