Roll20 is a virtual tabletop. You can upload images (maps, tokens, etc) to the tabletop and manipulate them. Roll20 is also system agnostic so we do not cater to any specific system. This means that, unlike some VTTs, we do not build game specific mechanics and rules into the VTT. We will try to accommodate different game requirements as they are suggested but we try to keep the automation low (just like a real tabletop). An example: A GM wants to GM Pathfinder using Roll20. He creates or copies maps and then loads them onto Roll20. He creates or finds and then loads tokens (miniatures) for his monsters and for the players to move around. He is now ready to run a game. If desired, he can use any number of other features but which ones he uses and how he uses them are up to him and his players. Virtually no rules get included into the Roll20 platform. The closest we have to a mechanic or rule is the measurement tool which is set up to accommodate multiple styles of measuring distances. - Gauss
