The answer to the question of what cancer is depends on your particular epistemological, clinical, and geographic training.  Chinese medicine explores cancer in a different way than does traditional western medicine.  From the Western perspective, Ellen Carr of Moores Cancer Center of the University of California, San Diego has created a wonderful introductory resource for understanding cancer.  This resource functions as a primer for understanding cancer.  I really like the way she integrates multiple choice questions that frame the important questions of the chapters.  It is a must-read for someone who is new to Cancer. The resource offers statistics, graphics, explanations, a glossary and covers topics ranging from cell biology, to cancer characteristics to a variety of therapies.  It is written clearly and is highly accessible to a general audience who has an interest in learning about Cancer. I have come back to it a few times to clarify my understandings.

In Traditional Chinese Medicine, there exists the concept of tumor, but not the construct of cancer itself. Using this clinical framework, tumors emerge from exposure to toxins, stressful emotional lives, unhealthy organs, and a gross accumulation of Qi.  I came upon this link for an accessible summary that made sense to someone who has grown up with Western Medical practices.