Although aloe has much in common with cacti, the two groups of plants belong to entirely different botanical families.
The approximately 400 species of succulent plants called aloe belong to the family Asphodelaceae and are native to Africa. The species are generally characterized by fleshy, succulent leaves with a waxy, water-retaining surface. The leaves vary in shape and size depending on the species, but they usually have spiny teeth long their edges. In some species, these teeth are rigid and relatively sharp, and in other species the teeth are soft and fleshy. The flowers of aloe are tubular in shape and are borne in clusters on upright flower stalks.
Aloe species are adapted to warm climates, but many of them are small enough to be grown in pots that can be taken indoors in areas where the plants wouldn't survive the winter. Other species are larger and can be grown as shrublike garden specimens in areas where winters are mild.
One of the most well-known aloe species is aloe vera (Aloe vera), a relatively small species that typically reaches a height of 1 to 2 feet. Its arching, pointed gray-green leaves can be up to 18 inches long, and the plant grows best when given access to full sun. It is winter-hardy and can be grown in gardens outdoors in U.S. Department of Agriculture plant hardiness zones 10 to 12, but it is commonly grown as a houseplant in colder climates.
Torch aloe (Aloe arborescens) is a much larger species that, in the right growing conditions, can reach heights of nearly 10 feet. It grows in clumps of succulent leaves with soft teeth long their edges, and it blooms with upright clusters of organge-red flowers. It is somewhat more cold-tolerant than aloe vera and is hardy in USDA zones 9 to 11.
The nearly 2,000 plant species considered to be true cacti are part of the family Cactaceae. Almost all of those species are native to the Western Hemisphere, and they are most common in regions with sparse rainfall. Although many species flourish in warm climates, some range as far north as southern Canada.
Most species of cactus have succulent, fleshy leaves and spines, and these characteristics often lead to confusion between true cacti and other spiny succulents such as aloe. Among the characteristics that differentiate cacti from other succulents are the presence of areoles, the budlike branches that produce spines and flowers. Cacti are also distinguished by flowers with combined sepals and petals, and numerous stamens and stigma lobes.