The common linking characteristic of cacti and succulents is the ability to store water in the leaves or stems enabling them to survive in arid habitats. All cacti are succulents, yet cacti are defined by the presence of areoles (specialised sites where spines form) whereas succulents have none.
The majority of cacti and succulents grow in desert and savannah situations with low moisture, dry air, bright sunshine, good drainage and high temperatures. However there are succulents such as Schlumbergera and Epiphyllum which grow as epiphytes in rainforests. These require semi-shade and humid conditions.