weak pawns

"The pawns are the soul of chess."

—Philidor

What is a weak pawn? A pawn that is exposed to attack and also difficult to defend is weak. There are several varieties: isolated, doubled, too advanced, retarded. An isolated pawn, because it is separated from the neighboring pawns and cannot be supported by them, requires defense by pieces. These pieces are called passive since they must function below their full potential. A doubled pawn is limited in mobility; its weakness is usually felt in the endgame because a group of pawns that includes a doubled pawn is less likely than a healthy group to produce a passed pawn. [However, not all doubled pawns are weak.] A pawn that is advanced too far runs the risk of being cut off from the rest of its army and becoming isolated. A pawn not advanced far enough [or not at all] can get in the way of the other pieces and may become backward and vulnerable.

source :: Reshevsky, Samuel [2002]. Art of Positional Play. New York: Random House.