Cacti are members of the family Cactaceae, and are a distinctive group of plants adapted for low-water and/or hot conditions. Most cacti have evolved so that the stem is succulent and photosynthetic, and the 'leaves' have evolved into spines.
Cacti are native to the Americas, with a range extending from Patagonia into southern, central Canada. They are at their most diverse in Mexico and tropical regions of Argentina & Bolivia. Note that this range includes areas that experience hard frosts - which some varieties tolerate without any problems. They have been introduced into other parts of the world for agriculture.
Cacti tend to have short growing seasons, long periods of dormancy, and large flowers. In the wild, most grow in mineral rich (humus poor) soils that are very free draining. The main exception are the epiphytic cacti which prefer organic-rich soils.
Cacti are usually grown as ornamental plants, mainly indoors, or in xeriscape applications.
They can also be grown for food (e.g. the fruit and pads of opuntia are edible when the spines have been removed), and fodder. Many opuntia plantations have been developed as a host for cochineal scale insects - used to make cochineal dye.
Gardeners are general wary of the spines, but they should also be wary of the hairs which often surround the areoles (spine bases). These are an irritant that are difficult to see when stuck in your skin. Even "spineless" cultivars of opuntia may have these hairs.
Use this tag for all questions about cacti. Use a specific species tag as well, where appropriate.