Plants that conserve water are called succulents, also known as "fat plants." They are called this because of their thick, fleshy leaves and stalks, which are designed to retain as much moisture as possible so that the plant might survive in extremely dry, arid conditions. Though cacti are first to come to mind, there are thousands of species, hundreds of genera, and many families of succulent plants across the world.
Cactaceae, as the name might suggest, is the family of cacti encompassing 1,600 species, all native to the Americas only. They grow only in arid, hot regions that receive very little water and lots of sunlight. As a result of the excess sunlight, they have no need for leaves and instead grow large stems from a central stalk. These stems can be anywhere from the thickness of a finger to the thickness of a barrel, and are composed primarily of fleshy chambers designed to retain water and use it only sparingly. As a defense mechanism most species of cactus has spines, the smallest of which can cause persistent skin irritation and the largest of which can easily skewer a human limb. Examples include prickly pear cactus, saguaro, barrel cactus, organ pipe cactus and Mexican lime cactus.
This is a family of plants encompassing roughly 600 species found in most desert and equatorial parts of the world. As a result, this encompasses plants used to tropical climates and rainfall, which are not technically succulents, and plants accustomed to desert climates, which are succulents. The majority of these plants' mass is made up of thick, angular, overlapping leaves that have tough exteriors and are often barbed. Tuberoses, cabuyo, agave, Spanish bayonet, and safed musli are succulent examples of this family.
Asphodelaceae is a family of plants native to Africa and Madagascar encompassing roughly 500 species. While considered succulents, they grow in areas with relatively high rainfall: marshes, rainforests, riverbanks, etc. What makes them succulents is their leaves, which are segregated into multiple chambers for moisture storage. While this family normally prefers areas of high rainfall, the segregated leaves give them the capacity to survive on their own water supply for months and even years at a time. This is believed to be an evolutionary mechanism which developed millennia ago to deal with disruptions in annual rainfall cycles, in which an area of land might not receive rain for months. Examples include aloe, kerry lily, poker plant, red hot poker, torch lily and bulbine.