Although it was not the first game of its type, Grand Theft Auto III would be considered by most to have started the wave of 'sandbox' games. You could even argue that the original GTA started the craze off, but it wasn't until the series gained a third dimension and highly publicised release on the consoles that the genre really hit its stride.

The defining characteristic of a sandbox game is that it provides an environment that appears to be its own living world. For a game with a contemporary setting, this means streets filled with cars and pavements swelling with pedestrians. Whatever the setting, these types of games will do their best to provide NPCs that appear to go about their lives regardless of what you, the player, might be doing.

The main supporting criteria of a sandbox game is generally that you are free to explore that living world outside of any normal mission structure. It is also common for these types of games to offer incentives for that exploration, either in the form of in-game rewards or some kind of trackable achievements.