Sometime after the release of Grand Theft Auto III, the popular term used in the game industry to describe open-world, mission-based games became "sandbox." It's kind of a dopey term, but it makes sense and properly conveys the sense of freedom that these games tend to have. But even with the freedom offered by this style of game, most of them tend to have a pretty rigid structure for you to follow if you want to see the game through to its storyline conclusion. Crackdown, from Microsoft and Real Time Worlds, tends to get away from that aspect. The result is an open-world game that feels more open ended than any other game of its type, but that lack of structure makes the game feel half finished and shallow in a few spots.