Isolated, backward or doubled pawns are often weak; however it is also true that sometimes they are not - other factors outweigh their potential weakness.
a doubled pawn gives its owner an extra open file for coverage of critical squares that would not be possible if the pawns were un-doubled and 'healthy'.
Said Brent Larsen of this game, "In this variation, many endings are won with the aid of the seemingly humble [doubled] center pawns."
A pawn is only weak if the enemy forces can get to it.
an isolated pawn is potentially weak because no other pawn can help defend it. Since now pawns can stand by its side, the square directly in front of it also tends to be vulnerable. However, it can control squares that might turn out to be useful homes for your pieces; there are open files on either side of an isolated pawn - a place for your rooks which can strive for activity.
An isolated pawn doesn't have to remain on a sickly square; it can advance and become a 'battering ram.'
If you advance and subsequently trade an isolated pawn, then the supposed weakness was nothing more than an illusion.
The most common type of isolated pawn tends to be the d-pawn.
Piece activity is key when trying to make use of an isolated pawn. Don't trade pieces when you have an isolated pawn - you need them to defend and advance the 'iso.' If you have an 'iso' and no minor pieces, try to swap rooks and queens.
the weakness or strength of a backward pawn depends on the following questions:
1. It is sitting on an open file?
2. How well is it defended?
3. Is the square directly in front of it adequately defended by pieces?
4. Is it serving a useful purpose defending the pawns that have gone ahead of it?
5. Can it successfully advance, thereby ridding itself of the 'backward' label?
If a backward pawn can safely advance then its weakness will turn out to be more illusory than real.
passed pawns are usually thought of as huge assets, offering great chances in an endgame, but they can also be a disadvantage.
You don't want a passed pawn if it can be blockaded, unless you have play elsewhere and the passed pawn acts as endgame 'insurance.' The best blockader of a passed pawn is a knight, since its strength is no diminished by having a pawn in front of it.