Most lung cancers are termed "carcinomas", which are tumors caused by uncontrolled reproduction of epithelial cells; these are the cells which form the linings and surfaces of most of the bodies organs and internal structures. There are two main types of carcinoma which are classified according to the size and presentation of the cancerous cells.

After a biopsy has been taken (which removes part of the tissue suspected to be cancerous), a laboratory specialist, known as a "histopathologist", places the biopsy sample under a microscope and is able to determine whether cancer is present and whether it is non-small cell or small cell cancer. The former accounts for the bulk of lung cancers (80%) but whatever classification is accorded, it is important because different treatment regimes are more effective at treating the different types (note that there are other classifications but these two are the major ones).

Non-small cell lung cancers (NSCLC) are themselves further classified into three different types though the treatment therapies are very similar for all of them:

Squamous cell lung carcinoma;

Adenocarcinoma; and

Large cell lung carcinoma.

Squamous cell lung cancer occurs in around a quarter of all lung cancer patients and is usually located near the central bronchus, this is the main airway leading into the lungs themselves. Fortunately, these types of cancer are less aggressive and grow slowly.

Adenocarcinoma occurs in approximately 40% of patients and starts in the outer lung tissue and is most closely associated with smoking, though many non-smokers also present this form of the disease. Whether a patient is male or female and whether they have smoked or not will determine different treatment paths for them under this category.

Small cell lung cancer (SCLC) is much less common and usually presents itself in the larger airways (the "bronchi"), but while less common they are unfortunately, much more aggressive. While this form lends itself to treatment by chemotherapy or radiation treatment, however, in many patients by the time it has been diagnosed the cancer has frequently spread (or "metastasized") to other parts of the body. The prognosis for SCLC is usually quite poor and is the type of lung cancer which is most commonly associated with smoking.

No matter what type of lung cancer a patient presents, it is vital that it is diagnosed as soon as is possible in order to maximize the chances of a positive outcome. Diagnosis and classification of the type of cancer is the first stage in the process of delivering effective treatment for patients, and as quickly as lung cancer has been diagnosed, it must then be "staged". Staging is the process by which the degree of spread and size of a cancer has gotten to; there are four stages, I through IV, with stage I being the best assessment and IV the worst. Staging is important because it directly impacts upon the treatment regime which must be delivered as well as classification of the cancer type. Being educated about your condition is key to choosing the right treatment.