Cranial nerves are nerves that emerge directly from the brain in contrast to spinal nerves which emerge from segments of the spinal cord. Although thirteen cranial nerves in humans fit this description, twelve are conventionally recognized. The nerves from the third onward arise from the brain stem.
Except for the tenth and the eleventh nerve, they primarily serve the motor and sensory systems of the head and neck region. However, unlike peripheral nerves which are separated to achieve segmental innervation, cranial nerves are divided to serve one or a few specific functions in wider anatomical territories.
The 12 pairs of cranial nerves are traditionally abbreviated by the corresponding Roman numerals. They are numbered according to where their nuclei lie in the brain stem, e.g. Cranial Nerve III (the Oculomotor nerve) leaves the brainstem at a higher position than Cranial nerve XII, whose origin is located more caudally (lower) than the other cranial nerves.
Cranial nerve zero (CN0 is not traditionally recognized.)
olfactory trigone, medial olfactory gyrus, and lamina terminalis
New research indicates CN0 may play a role in the detection of pheromones 
Provides motor innervation to the muscles of facial expression and stapedius, receives the special sense of taste from the anterior 2/3 of the tongue, and provides secretomotor innervation to the salivary glands (except parotid) and the lacrimal gland
Vestibulocochlear nerve (or auditory-vestibular nerve or statoacustic nerve)
Receives taste from the posterior 1/3 of the tongue, provides secretomotor innervation to the parotid gland, and provides motor innervation to the stylopharyngeus (essential for tactile, pain, and thermal sensation). Sensation is relayed to opposite thalamus and some hypothalamic nuclei.
Supplies branchiomotor innervation to most laryngeal and pharyngeal muscles; provides parasympathetic fibers to nearly all thoracic and abdominal viscera down to the splenic flexure; and receives the special sense of taste from the epiglottis. A major function: controls muscles for voice and resonance. Symptoms of damage: dysphagia (swallowing problems).
Provides motor innervation to the muscles of the tongue and other glossal muscles. Important for swallowing (bolus formation) and speech articulation.
Cranial nerves in non-human vertebrates
Human cranial nerves are evolutionarilyhomologous to those found in many other vertebrates.
Cranial nerves XI and XII evolved in the common ancestor to amniotes (non-amphibian tetrapods) thus totalling twelve pairs. These characters are synapomorphies for their respective clades. In some primitive cartilagenous fishes, such as the dogfish (Squalos acanthos), there is a terminal nerve numbered zero (as it exits the brain before the first cranial nerve).
As the list is important to keep in mind during the examination of the nervous system, there are many mnemonic devices in circulation to help remember the names and order of the cranial nerves. Because the mind recalls rhymes well, the best mnemonics often use rhyming schemes. The best known example is, "On Old Olympus' Towering Top AFinn And German Viewed Some Hops," where And represents auditory vestibular and Some represents spinal accessory.
Another to help remember the types of information these nerves carry (sensory, motor, or both) is thus:
Some Say Money Matters, But My Brother Says Big Brains Matter More.
For additional memonics, see b:Transwiki:List of mnemonics for the cranial nerves