Proxy Web Servers and Authentication > How Proxies Work > Cache headers

Entity

Headers which described an resource (can be used in either HTTP Requests or Responses).

ETag

The "Entity Tag" for the resource. Entity tags are unique identifiers for a specific version of an resource, and can be used with the "If-Match" and "If-None-Match" request headers to determine when an resource changes.

Expires

Specifies the date and time the entity expires. Cached copies of an entity should not be used after this time without revalidation.

Last-modified

Specifies the last modification date and time of the resource on the origin server.