How does cancer develop?

Cancer develops as a consequence of genetic damage in normal cells. The damage can be congenital (10-15 percent of cancer cases) or be caused by, for example, external factors. The cell contains genes which in turn contain germ plasma (DNA) that determines how the cell’s proteins are to be produced. Cancer develops when a string of mutations take place in genes that govern the cell’s ability to grow and divide. These mutations often lead to an overstimulation of cell growth. But the cells also contain tumour suppression genes which slow down an uncontrolled cell division. Similar slowing signals can originate from surrounding cells. Normally however it takes several years, even decades, for enough mutations to occur to develop a cell into a tumour cell. During this period the transformed cell is very brittle and weak.  During this period we can affect its future fate provided we know of its existence.
http://www.aicr.org/reduce-your-cancer-risk/tell-me-about/tellmeabout_what_is_cancer.html
For malignant tumour cells to develop into cancer tumours and for daughter tumours to gain a foothold and develop, a network of new blood vessels that provide them with nourishment must take form. The process is known as angiogenesis.
http://en.wikipedia.org/wiki/Angiogenesis

The cause of the cancer is often known and it is even possible to chart which genes have been damaged. Genes can be damaged by external factors, such as radiation, lifestyle factors such as nutrition and the environment, by toxins in the environment and by certain medications and viruses. The body’s own repairing mechanism for correcting incurred injuries, inflammation, is thought to favour cancer growth if it continues uncontrollably and hinders the body’s defence mechanism against cancer.
An important cause of cancer is aging. Mutations occur in all age groups but in the young the damage is repaired by certain enzymes. As you age these enzymes function more poorly which can lead to accumulated damage of the DNA.

How does cancer spread in the body?
A malignant tumour characteristically grows and forces its way into surrounding tissues. Not only can the tumour cause damage by its expansive manner of growth, it can also grow into other tissues and cause damage there as well. The malignant cells can also detach from the tumour and spread via bodily fluids, with lymph fluid to lymph nodes or through blood to remote organs such as the lungs, the brain and the liver and there form daughter tumours called metastases.