Though both agave and aloe are native to desert regions, they come from different areas of the world and different families in the plant kingdom. Agave is from the Agavaceae family and is native to North America (Mexico). Aloe, a member of the the lily family known as Asphodelaceae (Aloaceae), and is native to parts of Africa and Arabia.
Agaves range from 6 inches to 10 feet tall. The smallest is Agave parviflora, which has sharp pointy leaves with curled fibers on the ends. The Agave mapisaga, on the other end of the spectrum, grows to 10 feet tall and 10 feet wide.
There are more than 450 species of aloe, ranging from 2 inches across to tree height. Aloe vera, often grown as a houseplant and kept in kitchens, contains a gel that helps heal minor burns. Larger aloe, such as the 50 foot tall Aloe bainseii, has "a grotesquely thickened trunk and tapering branches that make it look like a character out of a Dr. Seuss story," according to The Book of Outdoor Gardening.
Many varieties of aloe have warmly colored red-orange, yellow or white flowers that replenish themselves throughout the plant's life. Aloe marlothii, for example, has side-leaning flowers that resemble red-hot flames blowing in the wind. It blooms year-round.
A few agaves have flowers, but most of these bloom only once in the entire life of the plant. Queen Victoria agave, Agave victoria regina, is one flowering variety of agave. It blooms only after the plant has lived a decade or more and only in summer. After blooming, the central plant dies but any "pups" outside the main plant will survive.
The leaves of the aloe and agave may look similar, but they are actually quite different. The agave has a fibrous leaf, with a vascular system of fibers running the entire length of each leaf. These fibers are used to make rope and string among other things. The leaves often last the entire life of the agave. Leaf tips are often sharp enough to cause puncture wounds on people. Bud prints (impression on the leaf of the leaf edge when a bud) are present.
Aloe, on the other hand, has leaves with a gelatinous interior. Fibers are not present. Bud prints are absent. Leaf tips generally do not cause punctures.
Another difference is the leaf margin. Agave has distinct, sharp teeth (straight or hooked) on its margin, with a colored line of demarcation. Agave attenuata has no marginal or terminal teeth.
Aloes have what appear to be teeth, but are actually only elongations of the leaf without a distinct line of demarcation.