GENERAL ABOUT CACTI
Description cacti - The name cactus derived from the Greek word kaktos, which means prickly plant. Cacti come in unusual shapes, sizes, textures and flower colours. Most have abandoned leaves and unusual body shapes to prevent loss of water. Some are ribbed or segmented and they may be covered with decorative spines, bristles or hairs.
Origin - South-western USA, Central and S. America, and Southern Canada.
Family - Cactaceae
Varieties - There are two groups of cacti - the desert ones (usually covered in spines), and the jungle or rainforest cacti (often without spines and most are epiphytes). There are some 200 genera and several thousand species. They are generally small but others, such as the old man cactus, can grow to 3m tall. Cacti are flowering plants and their shapes can be spherical, pillar, cushion or candelabra.
Cacti are succulents (water storing plants). There are other succulents as well, but not all succulents are cacti.
Why are they storing water?
The rainfall in the regions they naturally occur is not frequent, so they adapted to be able to survive long periods of dryness. So, 1st important rule: Cacti would not be the most happy plants if you don't water them at all, but they might die if you water them a lot (or as you water your other houseplants).