What does ... mean / What is a ... ?

V(E)TLA

Varnish (Extended) Three Letter Acronym. Because we ended up naming so many things ‘Vsomething’, VCL, VMOD and so one, somebody eventually started talking about all our ‘VTLA’ and somebody else pointed out that would have to be VETLA because it is four letters.

VCL

A VCL (program) is how you tell Varnish what it should do with your traffic. The VCL program is compiled into C source code, which again is compiled into a shared library for maximum performance.