People often use the terms succulents and cacti interchangeably, which is scientifically incorrect. Understanding the relation between the two will help properly distinguish them and help with identification.
Succulents are plants that store water in their stems, roots, and leaves. There are about 60 different plant families within the group of succulents, including aloe, haworthia, sedum, sempervivum, and of course, cacti. Cacti are fleshy plants that store water, making them part of this group. Therefore, all cacti are succulents.
Cacti are simply a family, or sub-category within the group of plants collectively known as succulents. They range from tall and thin to short and round, and they usually do not have leaves or branches. In order for a succulent plant to be considered a cactus, the plant must have areoles. Areoles are small, round, cushion-like mounds of flesh where spines, hair, leaves, flowers, and more grow from the cactus. Areoles are only present on cacti, not all succulents.
Some succulents are often mistaken for cacti because they have thorns or spines, but these characteristics do not automatically qualify a succulent as a cactus. The areoles are the key to distinguishing the two. Without areoles, the succulent cannot be a cactus. It seems pretty straight forward, right? Well, there is a small grey area when distinguishing succulents from cacti. Scientifically, cacti are considered succulents, yet some botanists and horticulturists categorize the two differently. Botanists categorize cacti as succulents, where as some horticulturists exclude cacti from succulents. We just wanted to cover all of the bases, but in general…all cacti are succulents, but not all succulents are cacti.