They are slender, lightly built birds with long, forked tails, narrow wings, long bills, and short legs. Most species are pale grey above and white below, with a contrasting black cap to the head, but some have dark plumage for part of the year.
Terns breed in noisy colonies and lay their eggs on bare ground with little or no nest material. Marsh terns make floating nests from the vegetation in their wetland habitats, and a few species build simple nests in trees, on cliffs or in crevices.
Most species feed on fish caught by diving from flight, but the marsh terns are insect-eaters, and some large terns may take small land vertebrates. Many terns are long-distance migrants, and the Arctic tern may see more daylight in a year than any other animal.