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An ETag is an opaque identifier assigned by a web server to a specific version of a resource found at an URL. If the resource content at that URL ever changes, a new and different ETag is assigned.

In typical usage, when an URL is retrieved the web server will return the resource along with its corresponding ETag value, which is placed in an HTTP “ETag” field:

ETag: "unique_id_of_resource_version"
The client may then decide to cache the resource, along with its ETag. Later, if the client wants to retrieve the same URL again, it will send its previously saved copy of the ETag along with the request in a "If-None-Match" field.

If-None-Match: "unique_id_of_resource_version"
On this subsequent request, the server may now compare the client’s ETag with the ETag for the current version of the resource. If the ETag values match, meaning that the resource has not changed, then the server may send back a very short response with a HTTP 304 Not Modified status. The 304 status tells the client that its cached version is still good and that it should use that.

However, if the ETag values do not match, meaning the resource has likely changed, then a full response including the resource’s content is returned, just as if ETag were not being used. In this case the client may decide to replace its previously cached version with the newly returned resource and the new ETag.