This document discusses HHVM, the Hip Hop Virtual Machine developed by Facebook as a runtime for PHP that aims to be faster than traditional PHP interpreters. It provides an overview of what HHVM is, a brief history, compatibility information, how it works by compiling code to bytecode using a just-in-time compiler, installation instructions for Debian/Ubuntu, Mac OS X and Windows, and introduces the Hack programming language that can be used with HHVM.