Risk is a simple wargame laid out on a map of the world, which is divided into different land territories to be conquered. The primary object of the game is world domination, or occupying every territory on the board and in so doing, eliminate all other opponents. Players control armies with which they attempt to capture territories from other players, with results determined by die rolls. Combat is a simple dice rolling affair that stresses attrition, and reinforcements are given to players who collect sets of cards.
Risk is considered 'the' classic wargame, and is generally the only real wargame (other than more abstract things like chess) known to the general public. It has remained in print continuously since its inception, and has been re-'imagined' in a variety of different configurations, such as Transformers Risk, various Star Wars Risks, Narnia Risk Junior, etc. At heart, though, it remains a simple game of rolling dice and conquering territory by eliminating one's opposing forces.
Risk was invented by French film director Albert Lamorisse and was originally released in France in 1957, as La Conquête du Monde (The Conquest of the World).