Varnish Cache is a web application accelerator also known as a caching HTTP reverse proxy. You install it in front of any server that speaks HTTP and configure it to cache the contents. Varnish Cache is really, really fast. It typically speeds up delivery with a factor of 300 - 1000x, depending on your architecture.
Varnish performs really, really well. It is usually bound by the speed of the network, effectivly turning performance into a non-issue. We've seen Varnish delivering 20 Gbps on regular off-the-shelf hardware.
One of the key features of Varnish Cache, in addition to it's performance, is the flexibility of it's configuration language, VCL. VCL enables you to write policies on how incoming requests should be handled. In such a policy you can decide what content you want to serve, from where you want to get the content and how the request or response should be altered. You can read more about this in our tutorial.
Varnish is written to run on modern versions of Linux and FreeBSD and the best experience is had on those platforms. Thanks to our contributors it also runs on NetBSD, OpenBSD and OS X.