The metaplot of the game involves a four-way war between the science-based Technocracy (Order), the insane Marauders (Chaos), the cosmically evil Nephandi (Entropy) and the nine magical Traditions, to which the player characters are assumed to belong.
The system is basically the same as in all Storyteller games. See White Wolf Dice Mechanics for a basic explanation.
Like other storytelling games Mage gives the players more freedom in comparison with conventional role-playing games, a power that can be used for deepening the story or simply to "win" more quickly.
Mage's unique point is the powerful magic system (even more free-form than Ars Magica). This poses challenges to players and gamemasters, since a large part of the game is about defining what the character(s) thinks the world actually is.
Mage has a reputation as the problem child of the World of Darkness line, with a freeform rule system, a vague and esoteric subject matter and a complex message that writers and developers complain has been ignored or misinterpreted by players. The third revision of the rules, Mage: The Ascension Revised, made significant changes to the rules and setting, and alienated many fans who stuck with their second edition rules.