Almost all Cacti are succulents, meaning they have thickened stems, leaves, or roots that store water and aid the plant in times of drought. Not all succulents are cactus.
Cactus, plants in the family Cactaceae, and are all native to the Americas ranging from Patagonia to British Columbia in Canada.
The one exception in Rhipsalis baccifera, which is also native to tropical Africa, Madagascar, and Sri Lanka, its seeds probably spread by birds long ago.
Most Cacti have evolved to survive periods of drought, though they do not all hail from the desert environments we commonly associate them with. For cacti adapted to drought, the three main centers are Mexico and the southwestern United States; the southwestern Andes, where they are found in Peru, Bolivia, Chile and Argentina; and eastern Brazil, away from the rains of the Amazon Basin. Tree-living epiphytic and climbing cacti have different centers of diversity, as they require moister environments. They are mainly found in the coastal mountains and Atlantic forests of southeastern Brazil; in Bolivia, which is the center of diversity for the subfamily Rhipsalideae; and in forested regions of Central America.
The spines that the Cactus family is famous for are actually specialized, reduced branches called areoles, and can help identify cactus as different from other species of succulent. Flowers also rise from the areoles, and are rarely self-fertile. There is a wide variety of flower forms and colors among cactus, though are mostly radially symmetrical (bilaterally symmetrical in some species such as the Christmas Cactus) with many stamens and only one style that may branch and end into more than one stigma.
The ribbing found on many cactus is actually an adaptation to allow expansion and contraction as the plant absorbs and stores water when available, saving it for dryer times when it shrinks and shrivels back.