Succulents are plants from a wide range of families with specially adapted leaves and/or stems that allow them to store water within the tissue. They tend to originate in areas where the water supply is erratic — the succulent tissue enables water to be stored when it is plentiful for use when it is scarce. Many of these plants have also developed a waxy outer covering to their leaves, or a rosette-forming habit to cut down on the amount of moisture lost. Some have adapted even more, and reduced their leaves to a bare minimum as spines.
Succulents are fairly easy to cultivate because they can withstand a degree of neglect, but they do need conditions to mimic their natural habitat: free-draining growing medium, good light, water during the growing season, and a cool, dry rest period.