VCL - Varnish configuration Language
The VCL language is a small domain-specific language designed to be used to define request handling and document caching policies for the Varnish HTTP accelerator. When a new configuration is loaded, the varnishd management process translates the VCL code to C and compiles it to a shared object which is then dynamically linked into the server process.
The VCL documentation can be found on the documentation site.
Users have contributed examples from their VCLs to solve common problems.