Pocket pairs from deuces through fives (22-55) can be considered small pairs, although opinions may differ on 55 and 66. Small pairs have a negligible expectation of winning unless they flop a set or straight draw.
Small pairs can be profitable to play in some games, but not in all games or from all positions. They do best in both loose and aggressive games. Loose games are preferrable because you can count on having enough opponents in the hand to pay you off if you hit (at least 5 in the hand is the generally-stated requirement); aggressive games are preferrable because aggressive players will be most likely to bet or raise into you, also resulting in your getting paid off. (This contrasts with small suited connectors, where you want the game to be loose and passive because your hand often won't be made on the flop.)
|PokerWiki's guide to limit hold 'em preflop play|