There are three basic mechanisms for controlling caches: freshness, validation, and invalidation.
Freshness allows a response to be used without rechecking it on the original server, and can be controlled by both the server and the client. For example, the Expires response header gives a date when the document becomes stale, and the maxage directive tells the cache how many seconds the response is fresh for.
Validation can be used to check whether a cached response is still good after it becomes stale. For example, if the response has a Last-Modified header, a cache can make a conditional request using the If-Modified-Since header to see if it has changed. The ETag (entity tag) mechanism also allows for both strong and weak validation.
Invalidation is usually a side effect of another request that passes through the cache. For example, if a URL associated with a cached response subsequently gets a POST, PUT or DELETE request, the cached response will be invalidated.
