Which vocalization do wolves use to communicate over long distances?

When you hear a wolf howl in the night – they are not howling at the moon – they are communicating. They call any time of the day, but they are most easily heard in the evening when the wind dies down and wolves are most active. Wolves’ vocalizations can be separated into four categories: barking, whimpering, growling, and howling. Howling is the one form of communication used by wolves that is intended for long distances. A defensive howl is used to keep the pack together and strangers away, to stand their ground and protect young pups who cannot yet travel from danger, and protect kill sites. A social howl is used to locate one another, rally together, and possibly just for fun.

Source: International Wolf Center