COPD is a chronic obstructive lung disease characterized by obstruction of airflow from the lungs. It encompasses emphysema, chronic bronchitis, and asthma. Emphysema involves destruction of lung tissue and abnormal enlargement of airspaces, while chronic bronchitis is defined by a cough producing mucus for at least 3 months per year. Smoking is the leading cause of COPD and causes an imbalance between lung elastase and alpha-1 antitrypsin, damaging lung elastic tissue over time and leading to airflow obstruction. Symptoms include shortness of breath, chronic cough, wheezing, and increased susceptibility to infection.