What's the difference between lung cancer and emphysema?

Big difference: Emphysema is damage to the alveoli, the oxygen transferring areas on the lung. Lung cancer is a mass of malignant cells but do not directly affect the alveoli. Lung cancer can affect breathing by blocking the airway. Hope this helps.

COPD versus cancer: In emphysema the air sacs of the lung (alveoli) are damaged. The result is the lung "traps air in the damaged sacs and it becomes hard to exhale. The lung becomes over inflated. Breathing (getting oxygen in and carbon dioxide out) are progressively harder. In lung cancer a cell is damaged and can not regulate its growth, ( ie becomes cancerous) with tumor formation. Both are linked to smoking.