Cancer is very complex and it is hard to explain fully how cancers arise. However, I can point to a great review article by Douglas Hanahan and Robert Weinberg entitled "The Hallmarks of Cancer," which goes over some of the common capabilities that cancer cells need to acquire in order to proliferate uncontrollably. The article points to six "Hallmarks of Cancer." First, the cells need to grow in the absence of a growth signal (often through activation of oncogenes). Second, the cells need to be unresponsive to anti-growth signals. Third, since the immune system targets abnormal cells, cancer cells need to evade the immune system by disabling apoptosis (programmed cell-death). Hallmarks two and three are often accomplished through inactivation of tumor-suppressor genes. Fourth, in order to feed themselves, cancer cells need to promote angiogenesis, the formation of new blood vessels. The combination of these factors and others (e.g. telomerase activation and inhibition of signalling through cell adhesion molecules) give cancer cells unlimited replicative abilities and the ability to metathesize (the fifth and sixth hallmarks of cancer).
Hanahan, D. & Weinberg, R.A. The Hallmarks of Cancer. Cell
100, 57-70 (2000).http://dx.doi.org/10.1016/S0092-8674(00)81683-9