Disclaimer: I work for Varnish Software. Since I was asked I'll still answer. Varnish was built for caching web apps. Squid is a forward proxy that can be configured to work as a web app caching program. read more
Varnish was built for caching web apps. Squid is a forward proxy that can be configured to work as a web app caching program. So, when Varnish was designed we where able to disregard a lot of stuff that isn't needed when caching in reverse mode. read more
In my opinion, this means Varnish is better suited for caching content for individual sites (by extensively tuning Varnish) and Squid is better for caching content for many sites (each of which will have to make their content "cachable" according to spec). read more
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. read more