A cat has three main types of cat hair and whiskers The hair types can overlap in appearance:
All cat hair molts. This happens all year long and seasonally (summer – it is the presence of light that kicks off the shedding). Molting affects males and females. The winter coat is slightly longer than the summer coat. The descriptions below apply to longhaired and shorthaired cats alike. The extended period of growth accounts for the longer hair. The slower rate of growth as opposed to period accounts for the ultra short coats of the rex cats.
Guard hairs These are the top coat with the awn hairs. These are normally straight and they taper to a straight point. These are the longest hairs. They constrict in diameter where the hair enters the skin. They thicken inside the skin (subcutaneously).
Awn hairs These serve as a protective layer to the soft down hairs. They also have a sensory function. These are thinner than guard hairs and thicken near the tip of the hair before tapering to a fine point. The thickening near the tip is called subapical swelling for the technically minded. These hairs vary the most of the three types of hair. Some are similar in thickness to the guard hairs and some are similar in thinness as the down hairs (awn-down hairs). They sometimes have an almost imperceptible the subapical swelling referred to above. These are the second longest hairs. They constrict in diameter where the hair enters the skin. They thicken inside the skin (subcutaneously).
Down hairs These are the undercoat. These serve as an insulating layer protecting against heat loss. They are the thinest and finest hairs of the three types. They are the same diameter throughout the length of the hair. They are undulating. These are the shortest cat hairs. This may in part be due to the crimping of these hairs. Crimping means cat hair that has become wavy. They constrict in diameter where the hair enters the skin. They thicken inside the skin (subcutaneously).
Whiskers They are called vibrissae. They have a very sensitive super tactile function. These are very special and I have built a short page on these hairs. See Cat Whiskers.
Source: Robinson’s Genetics for Cat Breeders & Veterinarians (4th Ed.)