Croup refers to an infection of the upper airway, generally in children, which obstructs breathing and causes a characteristic barking cough.
The cough and other symptoms of croup are the result of inflammation around the vocal cords (larynx), windpipe (trachea) and bronchial tubes (bronchi). When a cough forces air through this narrowed passage, the swollen vocal cords produce a noise similar to a seal barking. Likewise, taking a breath often produces a high-pitched whistling sound (stridor).
Croup usually isn't serious and most cases can be treated at home.