Rookie cards are the foundation of the hobby, and their popularity has withstood the test of time. The trick is to identify them without a price guide, and it's not always easy since some cards that say rookie card on them actually aren't.
Your sports knowledge can sometimes help, as in a lot of cases (especially in football and basketball) a player's rookie card is from his rookie playing season. When in doubt, assume the card is a rookie and look it up later to find out for sure.
Also, late-blooming athletes can have rookie cards that increase dramatically in value. More than any other kind of card discussed in this article, rookie cards are worth holding onto in the hopes that they may be worth something extra down the road.