Replacing old carpet can be a costly pain in the neck. That's why investing a little more money in durable carpet is usually a smart choice. The longer your carpet lasts, the longer you can wait before replacing it -- and the more money you'll save in the long term.
Durability has a great deal to do with density and twist, two specifications listed on the label. Density simply refers to how close together the strands of fiber are -- fibers per square inch. You can judge density by bending it back and seeing how much backing peeks through. Denser carpet tends to last longer because it withstands impact better. It also protects from dirt and stain by making it more difficult for particles to sink through it, keeping soils on the surface and easier to clean [source: Home Depot].
However, twist is the number of times a strand of fiber is twisted per inch. Carpet that has a high twist level of 4 or more will also be more durable and less likely to unravel. Carpet doesn't have to be dense to be durable, as long as it has a high twist level (and vice versa).
Don't confuse density and twist with face weight, which is the amount of fiber measured in ounces per square yard (as opposed to total weight, which includes the weight of the carpet backing). Face weight won't tell you a lot about the durability of a carpet without knowing the density and twist, as well.