VCDS (VAG-COM Diagnostic System) is a system that turns your PC into the equivalent of the VW factory tool. It permits access to all diagnostic capable modules in model years 1990-2008. With the tool you can read DTCs (diagnostic trouble codes commonly called fault codes) with a full description of the meaning of the code. You can perform Basic Settings and Adaptations. You can view live data from the sensors in the vehicle. You can log and plot that same live data. You can code modules, allowing you to take advantage of options built into the modules that the factory may not have set for your country. You can replace parts and perform the coding needed to replace those parts. On some modules, you can perform output tests to test components connected to that module. And there is more .....
Have you been to Ross-Tech's website? There you will find what you are seeking to know as Ross-Tech is the creator and developer of this product.
As to the best buy, you can look to Ross-Tech's distributors or you can look around on the web for others. Just beware: the interfaces sold on eBay are not genuine Ross-Tech product. The experience of some has been very poor. Others have had success with these aftermarket products. My point is - check out the seller before you spend any money.