Explain what is an ETag and how does it work?

An ETag is an opaque identifier allocated by a web server to a specific version of a resource found at a URL.  The ETag is a part of HTTP, the protocol for the world wide web and when the server reads the ETag from client request, the server can then tell whether to send the file (HTTP 200) or tell the client just to use their local copy (HTTP 304).

Category: Web Developer Interview Questions.