A Common (known as Normal in the OCG) card is identified by having a black (white for Spells, Traps, and Xyz Monsters) card name, and does not have any holofoil on the card name or image. Some common cards are short printed by Konami. A listing of all super short or short prints can be found in Beckett's Official Magazine.
For Booster Packs from "Legend of Blue Eyes White Dragon" to "Light of Destruction", packs always contained 8 Commons, from "The Duelist Genesis" onward, it is possible to have a pack with only 7 Commons, since it is possible to get a Rare and a Holo in the same pack; Duelists Packs and reprint packs contain a different amount of cards in each pack. Because they're so easy to find and are often much less powerful than rarer cards, commons are worth much less than other cards. That doesn't mean commons can't be powerful and valuable, though.