Eight-sided dice are typically used in roleplaying games, such as Dungeons & Dragons and Pathfinder, although they do have their uses in tabletop games and educational applications as well. For example, the Star Wars X-Wing Miniatures game uses custom eight-sided dice.
With the shape of an octahedron, there is no established pattern for how the numbers are arranged on the dice. Sometimes they are arranged so that the numbers on opposite sides add up to 9 (1 and 8, 2 and 7, 3 and 6, and so forth), but this practice varies by manufacturer, so you're sure to see d8's with the numbers arranged differently. Fortunately this has no effect on how the dice roll so variant numbering schemes on d8's will not make them any more or less fair.