Vitamin C is used by your body to make collagen, a protein substance that is used for all connective tissues in your body (bones, teeth, skin, & tendons). Collagen is also forms the scar tissue that heals wounds.
Vitamin C also helps your body guard against infection, aids in the absorption of iron, and is necessary for the production of thyroxin.
Humans are among an elite and random group of animals that do not produce their own vitamin C. Also included are the other great apes, fruit bats, the Bulbul bird, a species of trout, and the mighty guinea pig. No one really knows why we're so special.
We first isolated vitamin C from lemon juice in 1932.
Before the 1800s it was common for sailors on long sea voyages to come down with Scurvy, due to the lack of fruits and vegetables at sea. (Meat and biscuits stored better). The first nutrition experiment conducted on humans was done in 1747 by Dr. James Lind, a British physician. He took six pairs of sailors with scurvy and gave each pair either vinegar, sulfuric acid, sea water, orange, or lemon. The two pairs who got citrus fruits recovered.
50 years later, the British navy finally required all ships to carry enough limes to prevent scurvy in the crews. The term Limey was used to mock British sailors because of this.
Vitamin C has sometimes been shown to reduce the severity of the common cold (when taken in large doses at the first sign of symptoms), although studies have come to very different conclusions for no apparent reason; the usefulness of vitamin C for cold prevention/reduction may depend highly on the individual. It may also help prevent heart disease.
The RDA (US) for vitamin C is 60 mg per day for adults, 35 mg daily for infants. The Food Standards Agency (UK) recommends 40 mg per day for adults. It is possible that this is not the optimal level, and many people recommend higher doses. Overdoses may be hazardous, although even 1000% of the RDA would not cause an overdose. At about 5000 mg you may experience diarrhea; the LD50 is 11900 mg/kg. Other negative effects, including vitamin C being a possible carcinogen, are as of yet unproven.
You can get vitamin C from citrus fruits (lemon, lime, orange, grapefruit, etc.), dark green leafy vegetables (kale, mustard greens, spinach, arugula, etc.), and many other random fruits and vegetables (tomatoes, bell peppers, broccoli, potatoes; guava, papayas, strawberries, cantaloupe, mangoes, and many others). Cooking can destroy vitamin C, so eat these raw when you can. (However, this rule does not apply to potatoes. Cook those, but leave the skins on, as that is where the happy vC goodness lies.) Vitamin C is water soluble, so boiling can suck it out of foods; steaming or lightly simmering will reduce vitamin loss.
You can get enough vitamin C to get by on from meat, but only if it is eaten raw, as some Eskimos do (or once did, anyway) to get them through the plantless winter; they may also have gotten some vC though eating the half-digested contents of Elk's stomachs. It's better to just eat your fruit and veggies.
You can also buy spiffy little pills that let you bypass the cooking, chewing, and calories all together.
AKA Ascorbic Acid, or more specifically, L-ascorbic acid.