An ETag is an opaque identifier assigned by a web server to a specific version of a resource found at a URL. If the resource content at that URL ever changes, a new and different ETag is assigned.
Example: ETag : "C772C2C87B5BB8B21AD68B2308CA29BA"
This identifier should be stored with any response to GET calls and sent back up when you make the same GET call in the future.
You send this up by including a Request Header called "IF-NONE-MATCH"
If the resource has not changed since you requested it last you will receive a 304 Not Modified status code back.