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 and 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 and backward. Just like normal hairs, whiskers fall out and 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, he uses his whiskers to find his way around and avoid the object. The whiskers are able to detect slight changes in the air current around the object and 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 and Cornish Rex breeds have shortened curly whiskers.
The whiskers behind the wrist on the foreleg are to assist with tree climbing and contact with prey. 
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 and down from the face, the cat is relaxed and happy, when hunting or feeling aggressive the whiskers point forward and 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 and water bowls.