Varnish is a state-of-the-art, high-performance HTTP accelerator. Varnish is targeted primarily at the FreeBSD 6/7 and Linux 2.6 platforms, and takes full advantage of the virtual memory system and advanced I/O features offered by these operating systems.

Varnish web accelerators are targeted at the web site or web application owners. This type of it's installed in front of web servers and application servers and use a alot of techniques to improve performance to all users accessing the accelerated web sites or web applications. Sometimes referred to as a Reverse proxy.

These type of server side accelerators has the added benefit of off-loading transactions and connection managements from the web or applications servers and hence, reducing its CPU utilisation and increasing web server or application scalability to handle more users with less bandwidth.

Just to explain the reverse proxy, A reverse proxy or surrogate is a proxy server that is installed within the neighborhood of one or more servers. Typically, reverse proxies are used in front of Web servers. All connections coming from the Internet addressed to one of the Web servers are routed through the proxy server, which may either deal with the request itself or pass the request wholly or partially to the main web servers.

A reverse proxy dispatches in-bound network traffic to a set of servers, presenting a single interface to the caller. For example, a reverse proxy could be used for load balancing a cluster of web servers. In contrast, a forward proxy acts as a proxy for out-bound traffic. For example, an ISP may use a proxy to forward HTTP traffic from its clients to external web servers on the internet; it may also cache the results to improve performance.