Isolated pawns are where there are no white pawns at each side, and it depends on which color you are. If you were white, and there are no white pawns at the other file (which is right next your file)Both sides) for example:
As you can see here at the D-file there is one pawn and the files next to it there are no pawns. Whenever this happens it is called Isolated Pawn.
Passed Pawns is usually better in endgame. It is where it has a free path to get promoted that means none of the enemy pawns blocking your path way to get promoted for example:
As you can see here the D5 pawn is able to promote freely since there are no black pawns stopping it. This is called a passed pawn.
Doubled pawns is where two pawns are in the same file and this is usually bad because:
As you can see here C2 and C3 are in the same file. Now they are doubled, because they are doubled is ruining whites structure and if you look at black they do not have any doubled pawns that is why black's pawn structure is better then white's.
In this position the pawn D3 is preventing the knight to go to E4, and E4 is a superior square to the knight if it goes there. Both of the pawns are in the center. The D4 pawn is also ready to advance to D5 forking the the knight and the Bishop. If Cxd4 Then Nxd4 and there are no more double pawn. Well it is still good since now the knight got a good support point. This is clearly a winning position for white since it has the center under its control.
Backward pawns are where the pawn is the least developed, and cannot advance any further for example:
In this position the D3 pawn is the least developed and and cannot advance any farther. You might be thinking that the A2, B2, G2,and H2 pawns are least developed. That is right, but they can advance unlike the D-pawn which cannot advance so those pawns wouldn't be considered Backward pawns.