Named after the ubiquitous 20-sided die used in tabletop adventure games such as Dungeons & Dragons, Roll20 is a complicated beast. The online platform combines video chat, social networking, and numerous mapmaking and planning tools to help organize adventures for all kinds of role-playing games. Roll20 makes it possible for several people to participate in fantasy and science fiction gaming without being in the same place or owning a real-world copy of any books or charts or maps—or even those famous dice. With its group-finding system, it can even help players find new friends to game with.
Translating tabletop games to online systems is nothing new. These days nobody has to lug a stack of manuals to a buddy's basement for a fantasy adventure. What makes the Roll20 system special is that it's not actually a game, and it's not tied to any particular game or rule set. This is not like playing Scrabble online, or even Dungeons & Dragons online. Roll20 incorporates the rule sets of many different games (including D&D), but it also allows for complete customization, user-created content, and even a virtual marketplace. It has an app to use with tablets so you can play on the go. And it is just one example of widened possibilities in connective game playing and game creation, including, for more traditional card and tabletop games without a fantasy aspect, Berserk Games' Tabletop Simulator.