The name Cacti or Cactus has been adopted by the masses as the universal term to describe succulent plants. I have heard the term used to describe a varied range of plants from Euphorbias to Fouquierias (Ocotillo) to Agaves, Yuccas, & Aloes.

By including what are not cacti, CactiGuide.com can be a more useful guide through the process of elimination. The table below lists examples of other succulents that resemble cacti but are not. This should give beginners a good start. There are exceptions to every rule and this is so true with the features used to identify cacti. Those who happen across such an exception should prepare to do some extensive research and be patient. Some of the features that separate cacti from other genera are:

Agave	Typically large rosettes with long flat pointed leaves that terminate in an extremely sharp point. Commonly referred to as a Century Plant, Agaves can be found growing in many places that cacti grow. Agaves bloom once every 7-20 years or so and afterwards the main plant dies and is succeeded by off shoots or new plants at the flower tips.
Aloe	Aloe Vera is a name that everyone has heard before. Most people have used it to aid in healing a cut or a bad sunburn. Many lotions and soaps also contain Aloe. It seems people don't mistake Aloe Vera for a cactus, since they know it simply as "Aloe Vera", however, there are over 240 species of Aloe many of which are commonly misnamed "Cacti". Aloes occur naturally in the dry regions of Africa, Asia, Europe, and the Americas.
Echeveria	Most Echeverias are small rosettes that grow along rocky hillsides from the Southern United States south to Northern South America. A common name for this plant is "Hen and Chicks" and is widely cultivated in gardens and greenhouses around the world. Check to make sure that the cactus plant you are having a hard time identifying isn't an Echeveria.
Euphorbia	Perhaps no other taxon is more commonly confused with true cacti than that of Euphorbia. Some Euphorbias look more like a stereotypical cactus than even some cacti species. Euphorbias can be found all over the world, but most originated in Africa. If you cut a Euphorbia, it will secrete a sticky, milky-white fluid. This fluid contains latex. Flowers of Euphorbias are typically small nondescript buttons with no sepals or petals. Many Euphorbias also have thorns, making them appear even more similar to cacti. Upon closer inspection, however, one can easily see that these are not a separate spine arising from an areole like that of a true cacti.
Ice Plant	Another African native, the genus Lampranthus (Ice Plant) consists of a large number of species used very commonly as ground cover in warmer climates. These plants feature succulent leaves and spectacular flowers. Ironically Ice Plant stems most closely resemble the cactus species Rhipsalis, which exhibit the least exciting, relatively smallest flowers of the Cactus Family.
Ocotillo	Fouquieria is the genus commonly known as Ocotillo. Native to the Southwest United States and Mexico, Ocotillo grows along with cacti in these regions and even in areas that are too hot and dry for cacti. Ocotillo is confused with cacti because of where it grows and also because of its thorns. Ocotillo may or may not have leaves at any given time throughout the year. It may shed and re-grow leaves as much as seven times in one year.
Yucca	The top of a pineapple more closely resembles a Yucca than perhaps any other familiar object. Yuccas grow throughout North America in many of the places that cacti do. The stiff leaves of a Yucca resemble blades of grass arranged in a rosette, only they are very rigid with needle-sharp tips. Some Yuccas grow right on the ground while others have treelike trunks.
Lithops	These little "living stones" are another succulent often mistakenly referred to as a cactus. These unique and interesting plants hail from Southern Africa. They vary quite a bit in color and markings, but are all very similar in overal size and shape. Like the one shown here, they feature two stone-like leaves joined at a fissure. They range in size from 2 to 4 inches across.
Stapelia	Still a different Southern Africa native that has many people fooled by its succulent stems. Remembering that all cacti are succulents, but not all succulents are cacti, we can examine Stapelia and discover that they lack the qualifications of a true cacti. These plants are sometimes referred to as Star of David as the flowers resemble a Jewish star. If you're around this plant when its flowering you'll notice that it gives off an unpleasant odor which attracts flys for pollination.