Carpet underpads or underlays have to match the type of carpet. That means that if you have too soft an underpad under a carpet that needs a firm backing, you can destroy the carpet by bending it too much.
When an underpad has a gripping side and a smoother side, the gripping side goes against the floor.
Underpads are made out of all kinds of things, even old car tires. You may want to actually smell the underpad in the store before having yards and yards of the stuff spread out all over your house. Most have no smell at all, but some smell so bad I wouldn't want to have it in my house.
Carpeting is much like painting. The job is only as good as the primer coat, or in this case, only as good as the underpad. Don't try to put an expensive carpet over a cheap pad. That is not an economical combination. A good pad will stay put, will not bunch up, and will not flatten out with time. Always consult a carpeting expert to be sure you have chosen the right underpad for your carpet.
Thanks to Elite Carpets & Home in Toronto for all of our samples.