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In cellular model systems, cells are exposed to carcinogenic influences (chemicals, radiation). In these systems, the first signs of a cell developing into a tumor cell are:

Immortality. The usual number of cell divisions for a mammalian cell is 50-60 (cell senescence), then it ceases to divide. Tumor cells keep dividing forever.
Altered morphology.
Building of cellular clusters (Foci).
Loss of contact inhibition.
Low or no need for growth factors.

Items 2-4 (above) can sometimes be traced to mutations in genes that result in a disruption of cell adhesion. Some cell adhesion proteins are oncogenes.

Carcinogenesis (lit.: creation of cancer) is the process of derangement of the rate of cell division.

A cell that degenerates into a tumor cell does usually not acquire all these properties at once, but its daughter cells are selected to build them. This process is called cellular evolution. A first step in the development of a tumor cell is usually a small change in the DNA, often a point mutation, which leads, among other things, to a genetic instability of the cell. The instability increases to a point where the cell loses whole chromosomes, or has double ones. Also, the DNA methylation pattern of the cell changes, activating and deactivating genes more or less at random. Cells that divide at a high rate, such as stem cells, show a higher risk of becoming tumor cells than those which divide less or not at all, for example neurons. If the initial tumor cell (or group of tumor cells) is not removed by the immune system, it will develop into cancer.

AtCancer
Gives access to a MEDLINE-related search engine, as well as to reference links.
CancerSupportChat.org
Presents access to online discussion, chat and support.
Conversations In Care
Explores the challenges surrounding physician and patient communications in cancer.
MyOwnBiopsy
Provides "translations" of cancer pathology reports from medical jargon into understandable lay terms.
The "Y" Cancer Foundation
AOL discussion group aimed at providing information and support.

American cancer society