"They are water-retaining plants adapted to arid climates or soil conditions.
Succulent plants store water in their leaves, stems, and also in roots. In addition to succulence, succulent plants variously have other water-saving features.
Many succulents come from the dry areas of the tropics and subtropics, such as steppes, semi-desert, and desert. High temperatures and low precipitation force plants to collect and store water to survive long dry periods. Succulents also occur as inhabitants of sea coasts and dry lakes, which are exposed to high levels of dissolved minerals that are deadly to many other plant species.
The best known succulents are cacti (family: Cactaceae). Virtually all cacti are succulents, but not all succulents are cacti.
There is a significant difference between succulents that evolved in Africa and those that evolved in the Americas, the New World plants called cacti all have spines. No succulent plants arising in the Old World have spines although, through parallel evolution, there are similar species in the Old World that closely resemble species in the new world that do have spines.
The functional nature of the shapes evolved independently in each hemisphere, even though the specific structures may differ. The spines of cacti arose from leaf structures. This process may be confused with convergent evolution that occurs in species that are related more closely."
***Took everything from the link below:Source(s):http://en.wikipedia.org/wiki/Succulents