What is VCL?
Varnish Configuration Language (VCL) is a domain-specific language designed for use in Varnish.
VCL allows users to configure Varnish exactly how they see fit. It gives total control of content caching policies, HTTP behavior, and routing.
When a VCL configuration is loaded, Varnish translates the VCL code to C and compiles it to a shared object, which is then dynamically linked back into Varnish.
The Varnish Book is the bible of VCL. Download it here and learn to master the Varnish Configuration Language (VCL).