Varnish is an HTTP accelerator designed for content-heavy dynamic web sites as well as APIs. In contrast to other web accelerators, such as Squid, which began life as a client-side cache, or Apache and Nginx, which are primarily origin servers, Varnish was designed as an HTTP accelerator. Varnish is focused exclusively on HTTP, unlike other proxy servers that often support FTP, SMTP and other network protocols.

Varnish is used by websites including Wikipedia, online newspaper sites such as The New York Times, The Guardian, Gulf News, The Hindu, Corriere della Sera, social media and content sites such as Facebook, Twitter, Reddit, Spotify, Vimeo, and Tumblr.