A Common card is identified by having a black or white card name and the image is not holofoiled. Some common cards are short printed or super short printed by UDE. A listing of all super short or short prints can be found in Beckett's Official Magazine.
Each standard Booster Pack (Duelists Packs and reprint packs contain a different amount of cards in each pack) has 8 commons in it. Because they're so easy to find and are often much less powerful than rare cards (in fact, most commons are considered completely unplayable for one reason or another), commons are worth much less than other cards. That doesn't mean commons can't be powerful and valuable, though.