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Yes, that along with dealing with Last-Modified, If-Not-Modified, ETag, and other similar HTTP Headers is what is meant by HTTP Caching.

From what I can tell Varnish uses these same concepts to do what they do (caching HTTP reverse proxy).

See

* https://tools.ietf.org/html/rfc7234#section-1

* http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html

* https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching_FA...

Conditional GET and not re-downloading the same content are important aspects of caching. They save bandwidth and remove the download part of latency. For small resources, serving the content again instead of 304 will not matter.

Caching is simply keeping a response that hasn't changed so you don't have to recompute it. There can be many levels of cache. Varnish is one useful type of cache, but there are many others too.