Cancer is a class of diseases characterized by out-of-control cell growth, and lung cancer occurs when this uncontrolled cell growth begins in one or both lungs. Rather than developing into healthy, normal lung tissue, these abnormal cells continue dividing and form lumps or masses of tissue called tumors. Tumors interfere with the main function of the lung, which is to provide the bloodstream with oxygen to be carried to the entire body. If a tumor stays in one spot and demonstrates limited growth, it is generally considered to be benign.

More dangerous, or malignant, tumors form when the cancer cells migrate to other parts of the body through the blood or lymph system. When a tumor successfully spreads to other parts of the body and grows, invading and destroying other healthy tissues, it is said to have metastasized. This process itself is called metastasis, and the result is a more serious condition that is very difficult to treat.

Lung cancer is called "primary" if the cancer originates in the lungs and "secondary" if it originates elsewhere in the body but has metastasized to the lungs. These two types are considered different cancers from diagnostic and treatment perspectives.