Cactaceae are often times easily confused with succulents. They both possess leafless long lived fleshy stems of various shapes and sizes on which there are spines. A succulent is able to store water for survival in arid climates, as are cacti. However, the dichotomy between the two lies in one peculiar difference - Areols. Areols are highly modified lateral buds or dwarf shoots that produce spines and flowers in only cacti. This of them as the fuzzy parts on cacti where the spines emerge from.
Though succulents may be distantly related to cacti, they are indeed separate species endemic to distant regions of the globe. What is interesting, is though they both have evolved on opposite sides of the world, both cacti and succulents share very similar evolutionary traits. This is because of their similar latitudes on Earth. Cacti are native to the New World (South America). While succulents are only endemic to the Old World (Africa). The greatest confusion surrounds the Euphorbia family, in the family Euphorbiaceae.
Take a look at; Euphorbia Lactea, Euphorbia Obesa, Euphorbia Trigona or Euphorbia Abdelkuri and it becomes clear why it is so easy to get succulents mixed up as cacti.
The careful observer will note that though Euphorbia have spines that closely resemble cacti, they are paired in their dissimilar origin. This refers to the placement of the spines (spikes) on the plant. So it's the origin of the spines that set cacti apart from succulents. All true cacti have separate spines arising from the areole. Euphorbia on the other hand lack the presence of areoles.
One sure way to tell is to cut the plant. Euphorbia will all secrete a milky white substance. Be warned! This sap, known as latex can be poisonous, causes blindness, and is highly irritating. Mucous membranes like the eyes,mouth and nose suffer severe inflammation from contact. If surgery is not for you, the difference between succulent flowers vs cacti flowers is another reliable clue. Succulent flowers are not as flamboyant and showy as cacti, and can often go unnoticed. Small, clustered, and lacking pedals, they only have unisexual flowers- only pistil or only stamens. Cacti display large, colourful flowers with many segments. Cacti flowers harbour pistils and stamens together. This means some cacti can be self pollinated.
Test yourself next time when staring at what may look like a cactus. Looks can be deceiving.
All cacti are succulents but not all succulents are cacti.