Governs digestion and metabolism, also the regulation of body heat. Seated in the area of the abdomen.
Governs the movement of the musculature, also the circulation of energy in the limbs and extremities. Seated throughout the entire body.
Governs the vocal chords and vocal expression - speaking, singing, laughing, crying. Represents the upward-rising energies in the body. Seated in the throat and head.
These are the five major divisions of energetic flow in the body, according to Yoga. There are actually five more minor Vayus - Naga vayu for burping, Kurma vayu for blinking, Devadatta vayu for yawning, Krikala vayu (depending on whom you ask) for sneezing and/or hunger and thirst, and Dhananjaya (again, depending on whom you ask) governs decomposition of the body after death, and/or the opening and closing of the heart valves4. As you can see, there's less clear agreement on the functions of these minor vayus, and they're rarely referred to in the Hatha Yoga texts. The main Vayus, however - especially Prana, Apana, and Udana - are all of central importance to Hatha Yoga. Sometimes the early texts will specifically say something like, "This technique is important for activating Prana Vayu" - it really gets quite specific!
The word nadi is Sanskrit for "tube or pipe". Nadis are the pathways that carry prana throughout the body; they perform the same function for the movement of prana as do veins and arteries for the circulation of blood. The nadis of Yoga seem to correspond pretty well with the meridians of Traditional Chinese Medicine, as well as the sen lines of Thai Massage. Just as in
these other traditions, the nadis contain distinct points along their passageways which have particular energetic properties. Some of these energy points are more significant in their capacity and function than others - more about this below in the section on Chakras.
Most traditional texts mention there being a huge number of nadis in the body - the standard count is 72,0005, but it's not uncommon to see this count number in the millions6! It often seems that the authors of these early texts understood that these big numbers might be hard to believe, so usually the grand total is followed by
an admission that there are actually only a handful that are really important to know about for the practice of Yoga - usually just 147.
Out of those 14, the teachings continue, there are just three nadis which are considered to be the All-Out Most-Super-Duper Massively Important Nadis. (This is not an exact translation from the Sanskrit, but it's roughly equivalent.) These three are Ida, Pingala, and Sushumna.
The "lunar" channel governing the cooling, passive energy of the body. Corresponds with the parasympathetic nervous system.
Ida nadi is depicted as running up the left side of the body, roughly from the perineum up to the top of the head and then coming down the front of the head, ending at the left nostril. (Some sources have Ida nadi starting at the navel, some suggest that it begins at the left testicle/ovary instead.)