what’s the difference between Expires and Cache-Control headers?

Question

Cache-Control was introduced in HTTP/1.1 and offers more options than Expires. They can be used to accomplish the same thing but the data value for Expires is an HTTP date whereas Cache-Control max-age lets you specify a relative amount of time so you could specify "X hours after the page was requested".

HTML Cache control is a very similar question and has a good link to a caching tutorial that should answer most of your questions (e.g., http://www.mnot.net/cache_docs/#EXPIRES). To sum up though, Expires is recommended for static resources like images and Cache-Control when you need more control over how caching is done.

If you are using a CDN (Cloud Delivery Network) I recommend to use Cache-Control with a max-age time in seconds. For example Cache-Control: max-age=604800. This prevents request-peaks to your origin-server: With "Expires Wed, 30 Oct 20xx 04:37:07 GMT" all browsers will request you at the same time.

According to this Google Developers article, HTTP Caching:

Cache-Control header was defined as part of the HTTP/1.1 specification and supersedes previous headers (e.g. Expires) used to define response caching policies. All modern browsers support Cache-Control, hence that is all we will need.

Cache-Control was defined in HTTP/1.1, tells all caching mechanisms from server to client whether they may cache this object. It is measured in seconds: Cache-Control: max-age=3600.

The Expires header field gives the date/time after which the response is considered stale. The Expires value is an HTTP-date timestamp: Expires: Tue, 18 Jul 2017 16:07:23 GMT.

If a response includes a Cache-Control field with the max-age directive, a recipient MUST ignore the Expires field.

Heroku devcenter has an excellent article on this subject.

Quoting from it,

While the Cache-Control header turns on client-side caching and sets the max-age of a resource, the Expires header is used to specify a specific point in time the resource is no longer valid.