When pawns wind up one in front of the other, we refer to them as doubled pawns. Generally, doubled pawns happen after a capture has occurred and only a pawn can recapture. There are exceptions, but you should most often try to re-capture with a piece in order to avoid the weakness of doubled pawns.
As you can see in the following diagram, black has not one but two sets of doubled pawns. White's strategy becomes quickly clear. White will first double the rooks on the c-file. Black will have to respond by trying to the black rooks to defend the c-pawns rather than actively pursuing an attack upon the white pawns.
It is worth noting that you should generally not be in rush to attack such weaknesses. By all means do so if you see a clear win as a result, but bear in mind that weaknesses such as doubled pawns are "structural" in that the weaknesses will usually be there for the whole game. In most situations, you will be able to capture weak pawns once all of your pieces are well developed.