Cancer is the second leading cause of death in the U.S., and at any moment directly affects almost 4% of the population, or about 10.8 million Americans. A diagnosis of cancer can be one of the most frightening moments in someone's life, and yet most people understand little about the disease. I hear the same questions about cancer over and over again, so it's well past time to give a bit of an explanation of this set of diseases.
First of all, cancer isn't a single disease----it's over a hundred different diseases with certain commonalities. Second, cancer has many synonyms, some of which are rather confusing. A "tumor" and "neoplasm" can refer any growth, cancerous or otherwise. The modifiers "benign" and "malignant" tell you whether or not it's cancer. For example, a colon polyp is a benign neoplasm or benign tumor, whereas a colon cancer is a malignant neoplasm or tumor.
There is a lot of different and confusing terminology in oncology (the study of cancer) but in general, there is a pattern. Most cancers are named for the tissue in which they originated. As we get to terms, I'll define them.
So what makes these dozens of different diseases all "cancer"? Cancer cells are not normal. The growth and division of normal cells is closely regulated. If cells just kept dividing, we would all be very large, undefined blobs of tissue. Most cells in the body eventually die, either because they have been programmed to live only so long, or because they have developed genetic defects. If a cell develops a genetic defect which makes it forget to die, it stays where it is instead of making way for new cells. If it also develops a genetic defect that keeps it dividing, it not only lives for ever, but so do all of its daughter cells. A mass of identical, immortal cells that keep dividing is called a tumor.
A tumor, whether benign or malignant, can cause problems just by taking up space. For example, if a tumor grows in your throat, it doesn't need to be cancerous to prevent you from breathing. Still, cancerous tumors have some especially nasty properties. They not only take up space, but they actually invade nearby tissues, growing into them and destroying them. Going back to the tumor in the throat, a benign one will simply take up space, whereas a cancerous one could, for instance, grow into a blood vessel, causing you to bleed. The other nasty property of cancers is their ability to travel away from their origin. Most normal cells cannot just break away, travel through the blood, and make a new home elsewhere (metastasis). Cancer cells can often do this.
This property of metastasis confuses people quite a bit, because cancers are named for their tissue of origin. Someone with breast cancer with metastases to the brain does not have "brain cancer". They still have breast cancer, but with "mets" to the brain. Different cancers have different patterns of spread.
So, cancers are masses of immortal, dividing cells that do not respect tissue boundaries and can metastasize throughout the body.* They are named for the type of tissue and the organ they came from. This leads to further confusion. For example, there are many types of lung cancer. If the cells are glandular, the cancer is an "adenocarcinoma" of the lung. If they are from a bronchial lining, it is a "squamous cell carcinoma" of the lung. This system is not completely uniform, and the names of some cancers are hold-overs from an earlier era of medicine. For example, multiple myeloma is actually a type of blood cancer, closer to a leukemia than anything else, but the old name has stuck.
So why do we care about all these different names, etc.? Because different types of cancer behave differently. Cancers of the skin are the most common type of cancer, but most of them behave in a very benign way, making them easy to remove and cure. Cancer of the pancreas almost never behaves well.
When a cancer is suspected, a mass is biopsied, either by taking a small piece, or removing it altogether. A pathologist looks at the cells under the microscope and subjects them to various analyses to determine whether they are cancerous, what kind of cells they are, and whether they have begun to invade other tissues. The cells are often "graded" based on how abnormal they have become compared to their tissue of origin.
Once the cancer is identified, it is "staged", that is, the extent and severity of the cancer is defined, usually by the so-called TNM system (Tumor Node Metastasis). Using this collection of information---the cell type, grade, and stage of the tumor---and our knowledge of how tumors behave, treatment can be planned either to slow the cancer and/or make the patient more comfortable (palliative therapy), or to eliminate it (curative therapy).
Cancer is a blanket term for a set of diseases that involve abnormal cell growth. There is no one treatment for "cancer".** It is a complex, constantly changing field, and fascinating. And while "the C-word" may be scary, the details are important, and often a source of hope.
If someone tosses you the C-word, after the shock wears off, it's time to get to work. Find out what you're really dealing with. Demystify, gain knowledge, empower yourself. Cancer is usually a stressful journey, but not one that always ends poorly. Go and learn.
*Blood cancers don't usually form tumor masses (although they sometimes do). The tumor cells float through the blood along with normal cells. These cancers are usually called "leukemias" ("white blood"), and there are dozens of different varieties.
**The corollary is that websites and doctors that claim their product treats all cancers are full of shit. Run from them.
NB: There are many good and bad cancer resources on the web. A good place to start is the National Cancer Institute.