The Culture is a galaxy-wide civilization, so far advanced that it has solved most problems that afflict humanity. The great concerns of our time are all resolved. No longer planet-bound, no longer concerned with meeting needs; the Culture is a utopian, decadent paradise. A mix of wildly evolved humans and super-intelligent machines, including intelligent spaceships, it is very nearly all-powerful and omniscient.
But there are still parts of the galaxy, or at least parts of the Magellanic Clouds, where the Culture has not yet gained influence. Those parts of the Galaxy are the business of Contact, the part of the very loose government of the Culture that deals with alien civilizations. And in the difficult cases, Special Circumstances steps in to solve the problem. "Special Circumstances," like most names in Banks' books, is a euphemism: "Special Circumstances" isn't bound by the legal, moral or cultural constraints that bind the rest of the Culture.
Gurgeh, the protagonist, is recruited, perhaps blackmailed, by Special Circumstances to help Contact with an awkwardly difficult alien culture. The Azadians present a space-faring civilization, less advanced than the Culture but still powerful, whose entire ethos is based on The Game. Social position, military rank, governmental power, wealth; all of Azad is based on one's performance in The Game. Gurgeh is one of the Culture's best games players. Special Circumstances sends Gurgeh to Azad to compete in The Game.
At one level, Banks is writing about the effect of an advanced culture on a less advanced one. At another, he is having fun with a traditional space opera culture that is in contact with his more subtle and sophisticated one. At another, he is poking fun at traditional SF authors. Because as the story progresses, the underbelly of Azad is revealed to be disgusting and horrific; in some ways, the Culture's efforts to undermine Azad are morally justified.
But most of what Contact tells Gurgeh is a lie. He himself is an unknowing pawn in another game. When is it right to cheat? What is cheating? As ever, Banks asks the questions but doesn't really answer them, making you ask yourself instead, "Am I asking the right question?"
Banks' Culture is ironic and self-mocking. The intelligent ship that takes Gurgeh to Azad is the size of an asteroid but calls itself "Little Rascal." The equally vast ship that takes him back is named "So Much for Subtlety." But the Culture is deadly, too, as evidenced in _Consider Phlebas_, set a few hundred years earlier than _Player of Games_. The Culture is peaceful and principled; that doesn't mean non-violent or honest.
This is a very good book by a very good author. Banks never tells the same story twice, and in _Player of Games_ he sets a new benchmark for intelligent science fiction. Highly recommended.