vag i believe is short for volkswagon audi group , com for communication. Its just a usb cable with an obd2 connector, basically it allows communication between obd2<=>usb.
3 ways to tune your car
-go to a professional/shop that does tuning
-get an accessport and use the base maps or get the accessport and then get a custom ap map and use that.
-use a laptop and vagcom(05-06)/tactrix(05+) cable with a laptop and get a custom map.
Basically you can use the ap and not have to go to a tuner or get an etune, but those maps arent nearly as good as a custom tune($500) or etune($150). The last option will give you the same results but at the lowest cost. AP is for more convience and ease of use imo. But if you have a laptop and dont mind using it to datalog then I recommend that route. AP is handheld and some people prefer its straight forwardness.