Tumors are neoplasms (new growths) in the body. They can be benign (relatively harmless) or malignant (leading to death if untreated).
An example of a benign neoplasm (tumor) is the uterine fibroid. It is a benign growth of smooth muscle cells in the wall of the uterus (womb). They may cause pressure symptoms and discomfort (requiring surgical removal), but do not cause serious illness or death.
An example of a malignant neoplasm is lung cancer. The tumor usually starts in the bronchus (air tube) and spreads into the surrounding lung. Malignant tumors have a tendency to spread, both locally into adjacent tissue and lymph nodes, and distantly into other lung, liver, brain, skeleton, etc. Eventually, as the malignant tumors take over and destroy the function of vital organs, the patient will experience weight loss, eventual multisystem failure and death.
Cancer occurs when renegade cells from various tissue types in the body begin to multiply and grow independently of the body's normal mechanism of replacing dying cells. They take over and grow out of control, and eventually spread and begin growing elsewhere (metastasis). It is said that we all have little cancers starting from time to time, but our body defenses kill them off. It is only rarely that one of the little cancers gets out of control and takes over.
Many toxic substances are causes of cancer (carcinogenic), for example cigarette smoke and its adverse effect on the bronchial linings.
In terms of the rapidity of formation, most tumors grow fairly slowly. The average cancer might have a doubling time of weeks to months. So starting with a microscopic cancer, it may take months to years before it is large enough to see on x-ray. Then it may double in diameter in weeks to months, growing somewhat exponentially and seemingly more rapidly toward the end.
Some benign tumors take years to form, and never get very large.