The following is copied from Wikipedia:
VAG-COM is a third party Windows-based software package, produced by Ross-Tech, and used for diagnostics and adjustments of Volkswagen, Audi, SEAT and koda vehicles. These are four of the major brands produced by Volkswagen Group (VAG), where the software draws the first part of its name.
VAG-COM will perform most of the functions of a number of expensive tools available only to official dealers like the VAS-505x series diagnostic tools. In the past this has prevented owners and many small repair shops from performing some tasks such as diagnosing problems, diesel ignition timing, modification of convenience options such as automatic door unlocking, coding a replacement ECU or key to the vehicle and monitoring of many vehicle sensors for diagnosing problems. Unlike generic OBD-II/EOBD, this uses manufacturer-specific commands, which allows the user to access all diagnostic capable vehicle systemseven in vehicles which are not covered by generic OBD-II/EOBD (e.g. pre-1996). In general there are two ways to use this software, either in a package (software+hardware) distributed by the manufacturer, or, by building own interface hardware and using it with the public-available but limited shareware version of the software.
VAG-COM is also capable of interfacing with many of the cars and light trucks sold in the United States after January 1, 199
It's a very complicated bit of software which I have no experience of, but plenty on here have. Hopefully the above will help a little.