Ants are social insects which form small to large colonies. A typical colony contains an egg-laying queen and many adult workers together with their brood (eggs, larvae and pupae). Workers are by far the most numerous individuals in the nest. They are responsible for nest construction and maintenance, foraging, tending the brood and queen, and nest defence.

While all workers are female, they are sterile and do not lay eggs. Winged queens and males are present in the nest for only a short period. Soon after emerging they leave the nest to mate and establish new nests. Queens are generally similar to the workers, differing primarily in having larger bodies. In some species, fully winged queens are lacking and egg-laying is undertaken either by typical workers or by individuals which are morphologically intermediate between typical queens and workers (these are called ergatoid queens). Males are generally about the same size as the workers or smaller, and have smaller heads with large ocelli, very short scapes and small mandibles. In many cases males look more like wasps than ants.

Workers in a single nest can all be the same size or they can very greatly in size. When all are of the same or a similar size they are said to be monomorphic. In some cases the variation in size can be so extreme that large workers are twice the size of small workers (figures right). If variation between small and large workers is continuous, the workers are said to be polymorphic. If there are only two distinct size classes of workers, they are called dimorphic. Many of the polymorphic and dimorphic species show allometry. That is, the heads and mandibles of the large, or major, workers are disproportionally large when compared to those of the small, or minor, workers.

The workers are the most commonly seen caste, especially as they forage on the surface of the ground or when they are disturbed under rocks or other objects. However, in most species not all workers forage outside of the nest. Ants show a strong division of labour, where different workers perform different tasks within the nest, and in some cases the specific tasks undertaken will depend on the age of the ant.

It is common for young, newly emerged workers to remain in the nest and tend eggs, larvae and pupae. As the workers age, they may shift their activities away from tending brood and begin to undertake nest construction and excavation. Finally, later in life they may become foragers, leaving the nest to search for food. In contrast to this, some workers may perform the same activities throughout their lives, or in other cases, all workers may undertake all activities of the colony, performing any given activity for a few days before switching to another. In many dimorphic and polymorphic species, the size of the worker will influence its activities. For example, major workers may only be found in or near their nests while only minor workers forage away from the nest.

This cooperation and division of labour, combined with their well-developed communication systems, has allowed ants to utilise their environment in ways approached by few other animals.