A hymn is a religious song, especially one praising God. People often sing hymns during church services.
The word hymn comes from the Late Latin hymnus, "song of praise," and the Greek hymnos, "festive song in praise of gods or heroes." Though many people think of hymns as Christian, they're used in many other religions too, from ancient Greek hymns to deities, to Sikh hymns and Hindu hymns to many gods. What these songs have in common is giving thanks or praise to a god or gods.