The backward pawn is a pawn whose peers on the adjacent files have moved forward. This means that the pawn can not longer be defended by pawns, so if the opponent attacks it it must be defended by pieces. One more important point about the backward pawn is that if it can be advanced safely it will no longer be a backward pawn. For example if you move a pawn to e6 & c6 & being able to move the d7 pawn to d6 or d5 means that it will no longer be a backward pawn.
The backward pawn has all the disadvantages of an isolated pawn if it is on a half open file. But the same time it can not enjoy the same advantages that an isolated pawn offers, for example two half open files. So it can be a real weakness.
The owner of the backward pawn should try to control the square in front of the pawn and perhaps eventually aim to advance it to get rid of it. Another defensive strategy is to capture the opponent’s piece that is placed in front of the backward pawn in such a way that opponent has to recapture it with a pawn. In this case the backward pawn will no longer be on a half open file and can not be attacked so easily.
There are some strategic openings that are designed to create a backward pawn in the opponent camp. For example in the Queen’s Gambit Declined Exchange Variation White launches a minority attack and the position often leads to a Black backward pawn on c6. Here is a wonderful game featuring this theme:
In the following example, we will see how Karpov creates play around the opponent’s backward pawn on d6.