The major objective of reverse proxies is to put some functionality in between the request and the server/backend code. The functionality can be things such as SSL termination (HTTPS) or often caching.

Varnish is a HTTP proxy that is focused on caching. It gets the requests from clients and tries to answer them from its own cache. If Varnish cannot find a response from the cache, the request is forwarded to the backend server. Once the backend server responds, the answer can be cached before forwarding the same to the client. Varnish decides whether or not to cache the response based on the Cache-Control HTTP response header.

The performance benefit of Varnish can be quite dramatic, with response times often in microseconds.