Succulents are a wide group of plants that have evolved from ancient tropical forests to combat the feast and famine of water supply.
Succus is the latin for juice. Succulent plants have  stems,branches, roots, stalks, leaves or flowers that are juicy, fleshy or thick to retain moisture.
Parts of succulents above the ground act as reservoirs of water.
Succulents are varied plants not one or even related groups or families of plants.
All cacti are succulents but all succulents are not Cacti.
How do Succulents Preserve Water
Many succulents including cacti do not grow leaves reducing transpiration.
Thick leaves have space to store far more water than other leaves such as Aloe.
Some plants can be pulled underground to preserve water.
Minimum evaporation is achieved by hairy, spiney or waxy outer layers.
Shapes are developed to maximise retention and minimise water loss.
Some plants have roots that grow in bundles on the soils surface to capture any available moisture
Many Cacti breath oxygen at night and photosynthesis during the day.