When to push centrals pawns? Lets consider you have e4 d4 pawns with white or e5,d5 with black. When your opponent challenge your center with e5 or d5 with black, or with e4 or d4 with white of course his pawns are protected. How do you know if you have to push your pawn or simply capture the challenger pawn?
There are basically three possibilities:


Keeping the tension is often the best one: If your opponent resolves the tension by taking your pawn you can retake and centralise a piece and your remaining pawn often gives you a space advantage. 
Taking is usually the least attractive option, but it can be good, if you are the first (or only one) to take advantage of vulnerable squares. So you might take on e5 if you can bring a knight to d5, possibly because the black c-pawn is already on c5, whereas your c-pawn can still control d4. 
Pushing the pawn gives you a space advantage, but there is a big difference on which side of the board you get this space advantage! Pushing e5 gives you a space advantage on the king side. This can lead to an (piece-) attack against the king and is therefore a generally attractive option. It is not by random chance that against the French and against the Caro-Kann the e-pawn is often pushed to e5.
Pushing d5 gives you an advantage on the queenside and this is only attractive if you can follow it up with play on this side of the board. Therefore it is generally more often played if you already have a pawn on c4, which is ready to be pushed to c5. If you need too much time to get your queenside play going, there will be the counter lever f5, which gives black play on the kingside.