Cancer is a broad group of diseases characterized by the rapid, uncontrolled division and growth of abnormal cells. These cells can invade surrounding tissues and spread to other organs, a process known as metastasis, leading to severe health complications. With over 200 known types, cancer cells often form tumors, which can disrupt the body's normal functions.

Tumors are categorized into three main types:
* Benign tumors: These are typically non-cancerous, do not spread to other parts of the body, and are generally not life-threatening.
* Malignant tumors (cancer): These are cancerous and can invade nearby tissues and spread to distant organs through metastasis, posing a significant threat to life.
* Precancerous (or premalignant) lesions: These involve abnormal cells that have the potential to develop into cancer over time.

Common and often aggressive cancer types include lung, lymphoma, leukemia, prostate, breast, and colon cancer.