Believe it or not, disposable nappies are made from wood and plastic! The main body of nappies are made from pastics and are held together with glue. The clever part of a nappy is the absorbent pad which is made from a mix of wood pulp and 'super absorbent polymers'. The absorbent part is a chemical called Polyacrylate and is capable of absorbing 30 times its own weight of liquid. When Polyacrylate crystals absorb wetness, they expand into a gel which is how they manage to keep our baby's wetness in. Incidentally, this same material is used to create artifical snow and is sometimes added to soil to increase water absorption, particularly in the cultivation of potted plants.
Some nappies are made specifically for girls or boys and these are slightly more absorbent at the points that tend to get wettest - for boys this is at the front while for girls, this is towards the middle and back.
Disposable nappies have long been criticised for being environmentally unfriendly. These days they do break down in landfill but the process is still extremely slow but at least Polyacrylate prevents content from leaching from landfill sites which is one of the dangers of other disposed chemical products. The other point about the environmental impact of dispoable nappies is the amount of energy that they consume during manufacture and transport to market - far higher than for reusable equivalents.