Croup
Overview 
Croup refers to an infection of the upper airway, which becomes narrow, making it harder to breathe. Croup also causes a cough that sounds like barking.
The cough and other signs and symptoms of croup are the result of swelling and irritation around the voice box (larynx), windpipe (trachea) and bronchial tubes (bronchi). When a cough forces air through this narrowed passageway, the swollen vocal cords produce a noise like a seal barking. Taking a breath often produces a high-pitched whistling sound called stridor.
Croup
When the upper airway is infected with the virus that causes croup, tissues around the voice box (larynx) and the windpipe (trachea) will swell. When a cough forces air through this narrowed passageway, it may sound like a seal barking.
Croup most often occurs in younger children. It usually isn't serious. Most children can be treated for croup at home.