Density
Density refers to the thickness of the yarn and how close the tufts of yarn are spaced together. The higher the Density, the better the performance.

Twist
Twist refers to how tightly the pair of yarns in each tuft is twisted. It is expressed in a #/inch of length. The tighter the twist in the yarn, the higher the number, the better the performance.

Other factors affecting carpet durability are weight and pile height

Weight
Weight indicates how much fiber there is in the carpet, explaining why heavier carpets tend to cost more. Weight is a function of the pile height and density combined. Pile height is a personal choice whereas density is a truer measure of performance.

Pile Height
Pile height is the distance from the backing to the top of the tufts. Selection of pile height is based on the style and functionality you're seeking. 'Deep pile' is plusher, but can also show tracks and vacuum marks. 'Lower pile' height is firmer and may be preferred where wheeled furniture or toys are being moved around.