The family of dicotyledonous plants which includes such genera as Mammillaria (the nipple cacti), Opuntia (chollas), Ferocactus (barrel cacti), Escobaria (beehive cacti), and Carnegiea (saguaros). There are over 90 genera with around 2000 species amongst them.
They are generally fleshy/succulent because they store water in their modified stems. This is an adaptation for surviving in xeric environments as well as having reflective/defensive hairs and spines, and a growth habit which exposes as little surface area as possible to the midday sun.