A cactus is a plant that is a member of the family Cactaceae. This family is of course within the kingdom Plantae, but further more, Cactaceae is of the order Caryophyllales, which encompasses many families of flowering plants, many of which posses some of the features of cacti; succulent-type leaves for one, which reduces surface area and thereby reduces moisture loss through evapotranspiration. Some members of this order don't bear much resemblance to cacti at all, and their inclusion is often based on genetics.
Back to Cactaceae, the typical features of cactus serve important purposes. The absence of leaves (except in one of my favorite cacti, Pereskiopsis, though this trait hasn't developed in many other species as well), is an evolutionary adaptation to combat moisture loss in the arid environments in which these species have developed. Some tend to associate arid or dry, with hot, but this isn't always the case; there are many species of cactus which are in habitat in the northern mid-west of the United States, up into the Dakotas and further north, where it can get incredibly cold (check out the lophophora blog for some pictures of some of these such cacti in cultivation). These northern areas, while they can experience extreme cold and copious amounts of snow, are relatively arid (or dry, or lack much available water), and the latter condition is more or less typical in most cactus species' native habitats.
The arid climate which is home to many cactus has spurred the adaptation of, not only loss of leaves, but the development of ribs. Since the cactus has shed the leaves, and reduced its overall surface area to combat moisture loss, some may ask why not get rid of the ribs to reduce surface area even more? The answer is, because the ribs typical of most cactus (though less pronounced in some species), increase the plants ability to take in water when it gets it. The ribs, as you have noticed when you neglect your cactus and forget to water, will shrivel somewhat, but after you water it you will notice the ribs becoming full and firm again in the days that follow; the ribs can swell to hold moisture to get it through until the next rain (or the next time you remember to water it). Species from especially dry areas have developed a spherical shape (these are many of the species we prize, and that are widespread in collections, such as Astrophytum, Ariocarpus, Lophophora, etc.), which is the best possible structure for retaining moisture and limiting its loss (the most volume with the least surface area).
Another adaptation with the purpose of limiting moisture loss, though not visible to the naked eye, is known as Crassulacean Acid Metabolism. This process is complex, and it could have several articles of its own, but I will try to simplify it. Most plants, during the day, are constantly taking in carbon dioxide to use to carry out photosynthesis, and excreting moisture and oxygen in the process; this process results in a very high moisture loss through the plants pores (known as stomata or stoma), as the pores are left open to allow carbon dioxide in, but at the same time moisture laden air inside the structure escapes. Cactus and many other species of plants have found a way around this, through the Crassulacean Acid Metabolism, also known as CAM. Essentially, with CAM, the plant's stomata only open at night, when it is much cooler (and the air is often more moist at this time), to take in and store Carbon dioxide (and any moisture vapor that comes with it). During the day, the stomata close, and the plant uses the carbon it stored during the night to carry out photosynthesis. This could be the single greatest (or at least one of the top two or three) adaptations that allow cactus to inhabit the harsh environments they do.
Another characteristic that many cacti posses, spines, serve a couple main purposes. The first, and main purpose in my opinion ,as you would guess if you have ever been stuck by one, is to keep away herbivores (and humans probably). The second main purpose is to provide some protection from wind and sun, both of which can cause moisture loss. While we're on this topic, we should discuss a key part of cactus anatomy, the areole. This feature is specific to cacti, and it is from this small organ that spines, glochids (really nasty spines), flowers, and new growing tips come out of. This feature is indicative of a cactus; if you see an areole, its a cactus basically.
The root structure of most cacti consists of mainly fine feeder roots, an adaptation to limited availability of water (see my article The Best Cactus Soil for a little information on cactus root structure). These roots are typically shallow to uptake water before it can drain down through the soil, and can spread out a considerable distance around the base of the cactus, for essentially the same reason. Some cacti, such as many of the aforementioned spherical, or globose, cacti, posses large taproots. These act in a couple capacities for the most part. For one, they act as a further means of water storage, and a good one, since they are not exposed to the harsh sun and drying wind that the aerial portion is. They also stabilize the plant and help keep it from being uprooted, and new growing tips can spring forth if the head is damaged or removed, provided there is a sufficient portion of the plant intact. Then there are species of cacti, often called Jungle cacti (or epiphytic cacti), which climb, or grow primarily in trees, and develop roots anywhere along their stems where there is an adequate growing medium (much like ivy, briars, etc.).
Well I think this is going to have to be part of a series. This was just something I wanted to touch on, hopefully I can add more to this in the future, but I think we covered some good territory. I like to get back to basics from time to time, and every once and a while I learn something new or relearn something I forgot, hopefully you guys did to. Thanks, and stay tuned.
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