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.
To differentiate between these two basic types that seem so similar, but that are unrelated succulent plants, use of the terms, cactus or cacti, only should be used to describe succulents with spines.