Succulent plants are water-retaining plants adapted to arid climate or soil conditions. Succulent plants store water in their leaves, stems or roots. The storage of water often gives succulent plants a more swollen or fleshy appearance than other plants, also known as succulence. In addition to succulence, succulent plants variously have other water-saving features. These may include:
Crassulacean acid metabolism (CAM) to minimize water loss
Absent, reduced, or cylindrical to spherical leaves
reduction in the number of stomata
stems, rather than leaves, as the main site of photosynthesis
a compact, reduced, cushion-like, columnar or spherical growth form
ribs enabling rapid increases in plant volume and decreasing surface area exposed to the sun
waxy, hairy or spiny outer surface to reduce water loss via the creation of a humid microhabitat around the plant and a reduction in air movement near the surface of the plant.
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 in order to survive long dry periods. Succulents also occur as epiphytes, as such they have limited or no contact with the ground, and are dependent on their ability to store water. Succulents also occur as inhabitants of sea coasts, or salt pans which are exposed to high levels of dissolved minerals.
Reference : Wikipedia