HTTP cache
The HTTP cache stores responses for reuse without recomputation: in the browser, on the CDN, on the server. Cache-Control headers set the rules, resource by resource.
In practice
The standard strategy: hashed static assets cached immutably for a year, HTML in short cache or revalidated, APIs according to their sensitivity. The expensive mistake: replacing an image under the same filename — the CDN serves the old version for days. Versioning asset URLs solves the problem structurally; purging is only the cure.