Croup, characterized by a seal-sounding cough, is a condition caused by an acute viral infection in the larynx, trachea, and bronchi (hence its alternative name: laryngotracheobronchitis).
Viruses responsible for the infection include: parainfluenza (the most common factor), adenovirus, measles and respiratory syncytial virus (RSV). It causes the tissues in the upper airway to swell thus constricting the airway and causing a cough.
Croup typically affects children between the ages of 6 months and 5 years, although older children can get it, too. It affects boys more than girls and usually appears in the winter months. The barking cough is often preceded by mild cold symptoms (runny nose, mild fever, sore throat). Parents may also hear a slight crowing sound, referred to as "stridor," when a child inhales. This is a sign that the airway has become abnormally narrowed, and your child should be seen by a doctor.
Croup is usually worse at night and can last for 5 or 6 nights, with the first nights being the most severe. If symptoms last longer than a week or recur frequently, you should speak with your family physician.
Croup is contagious and can be spread through airborne droplets from sneezing or coughing, or from contaminated hands touching common surfaces (counter tops, door knobs, telephones, etc.).
In most cases, children with croup can be treated and managed at home with inhaling moist air (mist vaporizer or humidifier, or standing in steamed-up bathroom) and drinking lots of fluids. The child should be sat up during sleep, if symptoms worsen when lying down. Blocked nasal passages can be cleared using saltwater nose drops administered every few hours, followed by suctioning out of the fluid with a nasal aspirator or ear bulb syringe.
Children are most contagious within the first few days of onset of symptoms. They can return to school or day care once the fever has cleared. A productive cough may last another two weeks or so, but should not be a cause for keeping them at home.
Sources: http://kidshealth.org, www.mayoclinic.com, www.nlm.nih.gov, www.medicinenet.com, www.emedicinehealth.com