A cough is a sudden and often repetitively occurring reflex which helps to clear the large breathing passages from secretions, irritants, foreign particles and microbes. While useful to help clear the airways it can also be a sign of other disease and at the least a big nuisance!
The most common cause of cough is a simple viral infection - aka the common cold. However when a cough lasts too long (see below) it is called a chronic cough.
The most common causes of chronic cough are: post-nasal drainage (as seen with allergies or sinus infections), acid reflux, and asthma. However there can be many MANY other possible causes of chronic cough so its important to talk about this with your doctor.
When is a cough considered "chronic"? - great questions! There are various references that will say more than 2 weeks, more than 4 weeks, etc, etc. The way I see it - if its lasting more than what a typical cold
will and not improving, then it might be a good idea to ask your doctor want they think is going on. And be extra careful if your cough is associated with any breathing problems
and/or if you have asthma