Isolated pawn: a pawn with no friendly pawns on the neighbouring files. The d4 pawn is isolated. Its advantages are space in the centre and active pieces; its drawback is that it can't be protected by a pawn, so it is weak in the endgame.
The side with the isolated pawn shouldn't let the game get quiet, but open the position with the d5 break while the pieces are active. Play d5.
Doubled pawns: two pawns on the same file. They can't protect each other and are hard to advance. A backward pawn has fallen behind its neighbours and can't advance. Both are usually weaknesses.