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Broadly speaking, "communication" can be defined as any action taken by an animal (or plant, or other organism) that alters the behavior of another animal without using physical force. Communication happens when a sender passes information to a receiver, through a physical medium that can distort or degrade the message, or contain competing messages.
There are three broad categories of communication. Signals are where the sender actively tries to pass a specific message to specific receivers. Cues, such as the bright coloration of monarch butterflies that tell bird predators that they are poisonous and taste bad, are always "on" and take no effort on the part of the sender to send. Signs, such as footprints, aren't intended to communicate anything but nevertheless provide information that another animal can act upon.
Animals communicate for a variety of reasons. Some examples are to attract mates, threaten rivals, warn about danger, or share information about food sources. The different ways in which animals communicate can be visual, audial, or chemical (smell or taste). For highly social species, there may be facial expressions, body language, or dances.
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Visual Ways of Communicating
To attract mates, fireflies produce flashes of light. Males signal in flight, while females respond to males that they like from the ground or bushes. Each species has a different pattern so they can tell each other apart.
Cuttlefish are able to produce highly complex and detailed moving color patterns along the lengths of their bodies. A male cuttlefish can court a female with one side of his body and warn off rival males at the same time with the other.
Anoles are small, subtropical lizards that have brightly colored flaps of skin on the front part of the neck, called a dewlap. They use these in head-bobbing displays from high vantage points to attract mates and defend territories.
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Examples of Audial Communication
Male crickets chirp to attract mates and warn off rival males. They do this by rubbing the top of one wing against the comblike teeth along the bottom of the other.
Frogs are often surrounded by many other frogs that all croak during the same period, and must time their calls to fall between those of their neighbors if they wish to be heard. Each species of frog has its own call patterns.
Songbirds learn their species-specific songs in a way similar to human children learning language. There is a silent phase where the young bird must hear their species' songs, a babbling phase where they learn to produce the various syllables involved - often many more of them than necessary, and finally "crystallization" when they arrive at the finished adult song.
Many whales and dolphins are highly social animals with a variety of calls to communicate with one another. In some species, individual animals have "signature" calls that are like their names.
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Ants are highly social animals that form colonies with a sophisticated division of labor system. Ants use pheromones to communicate with each other, which they sense (along with other scents) with their antennae. Pheromones are used to mark trails to food sources, produce warning calls, identify individual ants and their tasks within the colony, and in some species for information warfare against rival ant colonies.
Musk is produced by adult male musk deer, and is probably used to attract mates.
A number of mammals use scents to mark the boundaries of their territories, like wolves, dogs, and cats. The way this is done is often in the form of urine, though some species have specialized glands that spray scents that are only used for communicative purposes.
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Bodies in Motion
Honeybees have highly sophisticated dances to signal other members of their hive on the location, quantity, and quality of food sources that they've found.
To attract mates, a variety of animals engage in courtship dances. Seahorses do so to strengthen pair bonds. Some reef fishes use courtship dances to coordinate pair spawning so that they can avoid predators. Many birds have courtship dances, such as peafowl. The peacock is well-known for his large, extravagant tail, which he spreads out in a large fan. Peahens then choose the male with the most impressive tail. Indirectly, this is a measure of the peacock's evolutionary fitness, for only the healthiest can make such large, expensive tails while not getting eaten by predators.
Animals that hunt in groups, such as lions and wolves, must communicate with each other to coordinate their actions as they bring down prey.
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Johnstone, Rufus A. 1997. "The Evolution of Animal Signals." p. 155-178 in Behavioural Ecology: An Evolutionary Approach. 4th ed., John R. Krebs and Nicholas B. Davies. Blackwell Science, Ltd. 456 pages.
Hauser, Marc D. 1996. The Evolution of Communication. Massachusetts Institute of Technology. 760 pages.