A Succulent is a plant that has a way to store water so it can keep growing without getting any rain for a long period of time, sometimes over a year or more. So what is a Cactus ? A Cactus is a Succulent that has groups of small bumps all over the skin called areoles. This is where the spines grow. Many cacti have a thick, tough skin (or epidermis) with fewer stomata (or breathing pores). In addition some are covered by a waxy or oily bloom. Both of these features helps to reduce water evaporation.
Some plants have tuberous roots for water storage.
Unlike most plants, cactus need very minimal amounts of water, thriving in well-drained areas. They store what water they do get, allowing them to survive droughts.
Thus, they are unusual