What are VMODs?
VMODs are Varnish modules that add new functionality to VCL.
A VMOD is a shared library written in C, which is loaded into Varnish and then callable from VCL.
VMODs are one of the factors that make Varnish and VCL so effective.
VMODs can introduce a wide variety of new features and functionality to Varnish, greatly increasing the power of VCL.
The Varnish Book is the bible of VCL. Download it here and learn to master the Varnish Configuration Language (VCL).