A rooster is a male chicken. If your cute baby chick grows up to crow loudly first thing every morning, he's probably a rooster.
The word rooster is an American invention, inspired by the rooster's habit of roosting, or standing up on a perch while sleeping or while on the lookout for danger. In the UK, a rooster is more likely to be called a cock or cockerel. In a flock of birds, the rooster's job is to defend the chickens and their nests from predators and other roosters.