There are over 500 different species of Aloe plants, grown around the world. It is a member of the lily family, and is part of a larger family known as the xeroids. Only four of the 500 different species have been shown to be of medicinal and nutrional value to people, and the Aloe Vera, Aloe barbadensis miller, is by far the best out of the four. It may also be known as the "lily of the desert", "plant of immortality", and "medicine plant". The name is derived from the Arabic alloeh, meaning "bitter".
The plant grows in a "rosette" formation, with the leaves growing up, then spreading out to the sides. This shape lets the plant shade parts of itself, and as the leaves have a slight concave curve, facing upwards, lets it catch as much rain as it can. The leaves, light to dark green in color, also have a waxy coating on the outside, helping to keep moisture trapped inside the plant. It can grow to incredible sizes, sometimes as large as 30 feet (so one source states), though I suspect these are offshoot colonies of the plant, looking like seperate plants. When it blooms, tall stalks grow up, covered in brightly colored coral flowers, and the nectar from these flowers is a favorite of hummingbirds.
Botanical and historical evidence points to the possibility that this plant originated in Africa, in warm, dry climates. It is a tropical plant, but one that has the ability to surive in a variety of air temperatures - as long as the ground does not freeze and kill the roots, the plant will survive - however, cold temperatures near or below freezing will cause damage to the leaves. It will also survive temperatures above 100 degrees Fahrenheit, and can survive severe drought. It thrives in humid jungles, though the root must not stand in water - the plant will drown.
The plant has been taken around the world because of it's usefulness and adaptibility. It is grown commercially in the Rio Grande Valley in Texas, in various areas in California and Florida, and in greenhouses designed for the Aloe Vera in Oklahoma.
The first document listing the Aloe Vera plant specifically was a Sumerian clay tablet, found in the city of Nippur, dating from 2200 B.C.E., where it was listed among plants with great healing power. An Egyptian document dating from 1550 B.C.E. gives the first detailed analysis of the medical uses, including twelve formulas for mixing the plant with other substances for both internal and external use. Around 400 B.C.E. the plant was exported to Asia, where it found many uses and became popular in India.
The Greeks first mention in the plant in the works of Celsius (25 B.C.E. - 50 C.E.), discussing the power of the plant as a purgative. Dioscorides (41 C.E. - 68 C.E.) traveled with armies, developing knowledge and skill at using Aloe. He gave the first detailed description of the plant, saying it's juices had "the power of binding, of inducing sleep", and that it "loosens the belly, cleansing the stomach". He noted the sap could treat boils, hemorrhoids, bruises, mouth irritations, and for problems with the eyes.
Around 700-800 C.E., the plant started being used in China, where it was called Lu-Hui, or "black deposit", or Hsiang-tqan. It was used by grounding the leaves and boiling them down, and the resulting product used for treating the sinus, and for fever and convulsions in children.
The two types of Aloe product are aloe vera "extract", and aloe juice. The extract is made by pulverizing whole leaves. Aloe juice is made from the gel on the inside of the leaf, and is often used to sooth irritations of the digestive system. Apparently, in some countries, such as Japan, the gel is commonly ingested in various forms, such as aloe yogurt. (Thanks gn0sis for the info.) Also, aloe vera juice makes a good hair conditioner and thickener (thanks karfung).
Extensive research on the plant since the 1930's has shown that the gel helps to heal wounds, ulcers, and burns. Not only does it create a soothing and protective coating, but it actually increases the healing rate.
Growing an Aloe Vera Plant
As the plant is about 95% water, it does not handle frost well. It should only be planted outdoors in areas that do not freeze (USDA zones 10-11). In any other areas, the plant should be grown in a pot. It should be kept in full sun, or light shade, and the soil should be at least somewhat fertile, and quick draining - the plant does not handle standing water well at all. While it can handle drought well, it should be watered. A potted Aloe should be moved outdoors during the summer for the benefit of the plant.
During the winter, the Aloe will become somewhat dormant. It should be watered only a cup or two at a time, and the soil allowed to become completely dry before being watered again. During the summer, the soil should be soaked, but then allowed to dry completely before watering again. A potted plant should be in a pot with a drainage hole, or have a couple inches of gravel at the bottom of the pot. The root system is shallow but extensive, so a more shallow, wide pot is preferable. The soil can be a commercial potting mix, with extra grit or coarse sand added, or a prepackaged cactus mix. It should be fertilized yearly, with a dilute (half-strength) 10-40-10 fertilizer.
Sources:
Aloe Vera Studies Organization, http://www.aloe-vera.org
About Aloe Vera, http://www.websettler.com/DianaDeone/why.html
Herbal Information Center - Aloe Vera - Herbs, http://www.kcweb.com/herb/aloevera.htm
Aloe Vera, http://seamonkey.ed.asu.edu/~storslee/aloe.html
Growing Aloe Plants, http://www.thegardenhelper.com/aloe~vera.html