What exactly is a Cactus?

[Dave]

A cactus (plural, cacti or cactuses) is a type of (usually) succulent plant belonging to the dicotyledonous flowering plant family, Cactaceae. The Cactaceae has (depending upon the authority) between 24 and 220 genera, with around 90 genera being the most widely accepted count, and with anywhere from 1,500 to 1,800 species. Cacti are well-known natives of the Americas, mostly in desert areas. Some are also rainforest epiphytes, growing on tree branches where, despite the high rainfall, water drains off quickly so that "dry" conditions prevail much of the time.