What is the difference between cactus and succulent? A cactus is the only plant that can sit in a blazing south window where the sun pours in, magnified through the glass. A succulent is any plant that stores water in juicy leaves, stems or roots in order to withstand periodic drought. Some people exclude fleshy plants such as epiphytic orchids, some include desert plants that are not fleshy (yuccas, puyas).

Cactus (cacti is the Latin plural of cactus) is simply a succulent that can store moisture but is placed in a separate category (Cactaceae). Conversely, not all succulents are cactus. Succulents do not belong to one plant family but are represented in over 40 botanical families that are spread around the world and include close relatives of the pointsetta, geranium, lily, grape, amaryllis, crassula, daisy, milkweed.  

Cactus comes from the word kaktos (spiny plant), used by ancient Greeks to describe a species that turned out to be not a cactus at all but a type of artichoke. 2000 years later Linnaeus (the plant classifier) adopted the name Cactaceae to embrace a group of plants whose peculiar traits included fleshy stems that served to store water, prickly or hairy coverings and few, if any, leaves.

You can easily identify cacti. With rare exception, they do not have leaves--as the result of their struggle to survive. They have stems modified into cylinders, pads or joints that store water in times of drought. Thick skin reduces evaporation. Most species have spines or bristles for protection against browsing animals, but there are some without spines; and several have long hair or a wooly covering instead.  Flowers are usually large and brightly colored. Fruit may be colorful and sometimes edible.

All cacti do have leaves in their seedling stage. And some sprout small leaves on new growth for a short time each spring. As changing climate conditions turned native habitats into deserts, most cacti gradually lost their leaves, which evaporated too much scarce water into the dry air. Instead, they began to store available water in their stems. Many can change their shape to adjust the area of their evaporation surfaces to varying conditions. Accordian-like ribs can expand when moisture is plentiful and contract during times of drought.

Most succulents (i.e. aloes, hawthorias, crassula, echeveria) evolved under less severe conditions than cacti in areas where rainy seasons were followed by long dry periods. Most have leaves. To tide them over the dry spells, their leaves gradually became fattened by water-storing tissues and their leaves became covered with a waxy or horny material that reduces evaporation from the surface.

The Cactus (Cactaceae) family ranges from Canada through Central America and the West Indies, and south to the cold areas of Chile and Patagonia (southern end of South America). Perhaps Mexico has the richest collection, but many are also found in the western deserts of the United States and high in the Cordilleras of Peru, Bolivia and Argentina.

Most succulents come from desert or semi-desert areas in warmer parts of the world (Mexico, South Africa). Some (sedums, sempervivums) come from colder climates where they grow on sunny, rocky slopes and ledges. Although the deserts of the world have many succulents, not all succulents are desert plants. They exist on mountains, in jungles and near lakes and seas. The semi-arid regions of North and South America, Asia and Africa all have succulents, but many also dwell in the rain forests. In the mountains, there are succulents that thrive despite bitter cold, strong winds and poor soil. Succulents and their native lands include: Aeonium: Africa, Canary and Madeira Islands; Agave: the Americas; Aloe: Africa, the Mediterranean, Atlantic islands; Cotyledon: semi-arid regions of Africa; Crassula: mostly Africa; Dudleya: coastal California and Mexico; Echeveria: the Americas; Faucaria: South Africa; Gasteria: South Africa; Hawthoria: South Africa; Kalanchoe: tropical regions of America, Africa and Southeast Asia; Sanseveria: Africa and India; Sempervivum: Central and Southern Europe, Asia Minor, North Africa.

http://ucanr.edu/sites/scmg/Top_Plants_Category_Parent/Succulents/