DEFINITION: Succulent plants, also known as succulents or sometimes water storage plants, are plants that have some parts that are more than normally thickened and fleshy, usually to retain water in arid climates or soil conditions. Succulent plants may store water in various structures, such as leaves and stems. Some definitions also include roots, so that geophytes that survive unfavourable periods by dying back to underground storage organs may be regarded as succulents. In horticultural use, the term "succulent" is often used in a way which excludes plants that botanists would regard as succulents, such as cacti. Succulents are often grown as ornamental plants because of their striking and unusual appearance.
The habitats of these water preserving plants are often in areas with high temperatures and low rainfall. Succulents have the ability to thrive on limited water sources, such as mist and dew, which makes them equipped to survive in an ecosystem which contains scarce water sources. Source - Wikipedia
Succulents are not to be confused with cactus; nearly all cacti are succulents but not all succulents are cacti.
Almost all cacti are succulents, meaning they have thickened, fleshy parts adapted to store water. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade. In the absence of leaves, enlarged stems carry out photosynthesis.
Cactus spines are produced from specialised structures called areoles, a kind of highly reduced branch. Areoles are an identifying feature of cacti. As well as spines, areoles give rise to flowers, which are usually tubular and multi-petaled. Many cacti have short growing seasons and long dormancies, and are able to react quickly to any rainfall, helped by an extensive but relatively shallow root system that quickly absorb any water reaching the ground surface. Cactus stems are often ribbed or fluted, which allows them to expand and contract easily for quick water absorption after rain, followed by long drought periods. Source - Wikipedia