Other than the fibre itself, there are three qualities of a carpet that will determine its overall durability. Together they contribute to the overall weight of the carpet. Heavier carpets tend to cost more, but will retain their appearance for longer than cheaper, lighter weight carpets.
Density is a measure of how thick each yarn is and how closely the tufts are spaced. This may be measured in stitches per inch, and a greater density will allow the carpet to resist crushing. If you can see the backing when you bend the carpet, it has low density.
Twist refers to how tightly the filaments in each tuft of yarn are twisted together. This carpet quality is measured per inch, where the greater the number, the more twisted the yarn is and therefore the less susceptible the carpet is to crushing and matting.
The pile height, or nap, is the distance between the backing material and the top of the tufts of yarn. A deeper pile will provide a plush look, but is prone to crushing so that tracks can be left in the carpet. A shallower pile looks less luxurious but is firmer so will retain its appearance.