Also known as 'vibrissae' or tactile hairs, feline whiskers are specialised hairs which are found on either side of the muzzle (sinus hairs or Mystacial whiskers), the cheeks,  above the eyelid, and on the wrists of the foreleg. The whiskers on the muzzle are the longest of the three sets on the face.

Whiskers  are two to three times the thickness of ordinary hairs and are embedded in the tissue of the cat's upper lip to a depth three times greater
than other hairs. They have a rich supply or nerves & blood vessels. Unlike normal hairs, the whiskers on at the side of your cat's mouth are surrounded by a highly developed sheath of muscle tissue which enables them  to move both forward & backward. Just like normal hairs, whiskers call out & are replaced with new ones.

Whiskers are the same width as the cat's body, which is useful for the cat to determine if it is wide enough to squeeze into a narrow space.

As the cat moves in the darkness, it uses it's whiskers to find it's way around & avoid object. The whiskers are able to detect slight changes in the air current around the object & the cat is able to walk around the object.

Whiskers are an extremely valuable tool for the hunting cat, especially at night acting as a guidance system. They are able to provide information on the outline of the prey, which enables the cat to bite the prey in exactly the right spot in order to kill it instantly. A cat with damaged whiskers will often aim the bite in the wrong area, therefore not killing the prey.

It's not uncommon for a mother cat to chew off the whiskers of her kittens.

The Devon & Cornish Rex breeds have shortened, curly whiskers.

The whiskers behind the wrist on the foreleg are assist with tree climbing & contact with prey. [1]

Whiskers also give a clue as to your cat's mood. Whiskers pulled tightly back on the face indicate an angry cat, when they are pointing slightly forward & down  from the face, the cat is relaxed & happy, when hunting or feeling aggressive the whiskers point forward & are tense.

Whiskers can be a bit of a nuisance when the cat is eating if the bowl is too narrow as they will touch the sides of the bowl, causing discomfort. Therefore it is a good idea to provide your cat with wide food & water bowls.