The barn swallow is the most widespread species of swallow in the world, occurring on all continents, with vagrants reported even in Antarctica. It appears to have the largest natural distribution of any of the world's passerines, ranging over 251 million square kilometres globally. It is a distinctive passerine bird with blue upperparts and a long, deeply forked tail. In Anglophone Europe, it is just called the swallow; in northern Europe, it is the only member of family Hirundinidae called a "swallow" rather than a "martin".

The swallows, martins, and saw-wings, or Hirundinidae are a family of passerine songbirds found around the world on all continents, including occasionally in Antarctica. Highly adapted to aerial feeding, they have a distinctive appearance. The term "swallow" is used as the common name for Hirundo rustica in the UK and Ireland. Around 90 species of Hirundinidae are known, divided into 19 genera, with the greatest diversity found in Africa, which is also thought to be where they evolved as hole-nesters. They also occur on a number of oceanic islands. A number of European and North American species are long-distance migrants; by contrast, the West and South African swallows are nonmigratory.