The name army ant (or legionary ant or marabunta) is applied to over 200 ant species in different lineages. Because of their aggressive predatory foraging groups, known as "raids", a huge number of ants forage simultaneously over a limited area.

Another shared feature is that, unlike most ant species, army ants do not construct permanent nests; an army ant colony moves almost incessantly over the time it exists. All species are members of the true ant family, Formicidae, but several groups have independently evolved the same basic behavioural and ecological syndrome. This syndrome is often referred to as "legionary behaviour", and may be an example of convergent evolution.

Most New World army ants belong to the genera Cheliomyrmex, Neivamyrmex, Nomamyrmex, Labidus, and Eciton. The largest genus is Neivamyrmex, which contains more than 120 species; the most predominant species is Eciton burchellii; its common name "army ant" is considered to be the archetype of the species. Most Old World army ants are divided between the tribes Aenictini and Dorylini. Aenictini contains more than 50 species of army ants in the single genus, Aenictus. However, the Dorylini contain the genus Dorylus, the most aggressive group of driver ants; 70 species are known.^{[citation needed]}

Originally, some of the Old World and New World lineages of army ants were thought to have evolved independently, in an example of convergent evolution. In 2003, though, genetic analysis of various species suggests that several of these groups evolved from a single common ancestor, which lived approximately 100 million years ago at the time of the separation of the continents of Africa and South America, while other army ant lineages (Leptanillinae, plus members of Ponerinae, Amblyoponinae, and Myrmicinae) are still considered to represent independent evolutionary events. Army ant taxonomy remains in flux, and genetic analysis will likely continue to provide more information about the relatedness of the various taxa.

The workers of army ants are usually blind or can have compound eyes that are reduced to a single lens. There are species of army ants where the worker caste may show polymorphism based on physical differences and job allocations; however, there are also species that show no polymorphism at all. The worker caste is usually composed of sterile female worker ants.

The soldiers of army ants are larger than the workers, and they have much larger mandibles than the worker class of ants, with older soldiers possessing larger heads and stronger mandibles than the younger ones. They protect the colony, and help carry the heaviest loads of prey to the colony bivouac.

Males are large in size and have a large cylindrical abdomen, highly modified mandibles and uncommon genitalia not seen in other ants. They have 13 segments on their antennae, are alate (have wings) and therefore can resemble wasps. Males are born as part of a sexual brood. As soon as they are born, they will fly off in search of a queen to mate with. In some instances where males seek to mate with a queen from an existing colony, the receiving workers will forcibly remove the wings in order to accommodate the large males into the colony for mating. Because of their size, males are sometimes called "sausage flies" or "sausage ants."

Colonies of real army ants always have only one queen, while some other ant species can have several queens. The queen is dichthadiigyne (a blind ant with large gaster) but may sometimes possess vestigial eyes. The queens of army ants are unique in that they do not have wings, have an enlarged gaster size and an extended cylindrical abdomen. They are significantly larger than worker army ants and possess 10–12 segments on their antennae. Queens will mate with multiple males and because of their enlarged gaster, can produce 3 to 4 million eggs a month, resulting in synchronized brood cycles and colonies composed of millions of individuals all related to a single queen.