Although the disease has an unknown etiology, the sores of angular cheilitis may become infected by the fungus Candida albicans (thrush), or other pathogens. Studies have linked the initial onset with nutritional deficiencies, namely vitamin B Riboflavin\[1] and Cyanocobalamin B12 and iron deficiency anemia, which in turn may be evidence of poor diets or malnutrition (e.g.) celiac disease Cheilosis may also be part of a group of symptoms (upper esophageal web, iron deficiency anemia glossitis, and cheilosis) defining the condition called Plummer-Vinson syndrome (aka Paterson-Brown-Kelly syndrome).Angular cheilitis occurs frequently in the elderly population who experience a loss of vertical dimension due to loss of teeth, thus allowing for over-closure of the mouth.

A cold sore at the corner of the mouth is a type of angular cheilitis.

Less severe cases occur when it is quite cold (such as in the winter time), and is widely known as having chapped lips. This lesser form mostly happens to young children/teenagers The child may lick their lips in an attempt to provide a temporary moment of relief, only serving to worsen the condition.

source wikipedia