According to American Cancer Society:

Cancer is the general name for a group of more than 100 diseases. Although there are many kinds of cancer, all cancers start because abnormal cells grow out of control. Untreated cancers can cause serious illness and death.

How normal cells act

The body is made up of trillions of living cells. Normal body cells grow, divide to make new cells, and die in an orderly way. During the early years of a person’s life, normal cells divide faster to allow the person to grow. After the person becomes an adult, most cells divide only to replace worn-out or dying cells or to repair injuries.

How cancer starts

Cancer starts when cells in a part of the body start to grow out of control. Cancer cell growth is different from normal cell growth. Instead of dying, cancer cells continue to grow and form new, abnormal cells. Cancer cells can also invade (grow into) other tissues, something that normal cells can’t do. Growing out of control and invading other tissues are what makes a cell a cancer cell.

Cells become cancer cells because of changes to their DNA (deoxyribonucleic acid). DNA is in every cell and it directs all its actions. In a normal cell, when DNA is damaged the cell either repairs the damage or dies. In cancer cells, the damaged DNA is not repaired, but the cell doesn’t die like it should. Instead, the cell goes on making new cells that the body doesn’t need. These new cells all have the same damaged DNA as the first cell does.

People can inherit abnormal or faulty DNA (it’s passed on from their parents), but most DNA damage is caused by mistakes that happen while a normal cell is reproducing or by something in the environment. Sometimes DNA damage may be caused by something obvious like cigarette smoking or sun exposure. But it’s rare to know exactly what caused any one person’s cancer.

In most cases, the cancer cells form a tumor. Over time, the tumors can invade nearby normal tissue, crowd it out, or push it aside. Some cancers, like leukemia, rarely form tumors. Instead, these cancer cells involve the blood and blood-forming organs and circulate through other tissues where they grow.

How cancer spreads

Cancer cells often travel to other parts of the body where they can grow and form new tumors that crowd out normal tissue. This happens when the cancer cells get into the body’s bloodstream or lymph vessels. The process of cancer spreading is called metastasis.

No matter where a cancer may spread, it’s always named based on the place where it started. For instance, colon cancer that has spread to the liver is called metastatic colon cancer, not liver cancer. In this case, cancer cells taken from the liver would be the same as those in the colon. They would be treated in the same ways, too.

How cancers differ

Different types of cancer can behave very differently. For instance, lung cancer and skin cancer are very different diseases. They grow at different rates and respond to different treatments. Therefore, people with cancer need treatment that’s aimed at their kind of cancer.

Tumors that are not cancer

A tumor is an abnormal lump or collection of cells, but not all tumors are cancer. Tumors that aren’t cancer are called benign. Benign tumors can cause problems – they can grow very large and press on healthy organs and tissues. But they can’t grow into (invade) other tissues. And they can’t spread to other parts of the body (metastasize). These tumors are seldom life threatening.