Oncogenes are Mutant Forms of Normal Genes
In nearly all of the trillions of cells that make up our bodies, the genes that could help tumors form, grow, and spread simply make proteins that help in normal cellular processes. In this normal state, they are called proto-oncogenes. The proteins produced by proto-oncogenes are usually very important in cells' growth and division. Thus, a mutation in one of these genes can throw off tightly regulated growth mechanisms. When a proto-oncogene is mutated in this fashion, it is then referred to as an oncogene.
The mutation which creates an oncogene can come in several different flavors, but all have one key quality: they increase the impact of the protein produced by the oncogene in the cell. In some cases, a regulatory component of the protein is defective or deleted. In others, the gene itself is normal, but it's been duplicated in the cell's genome, resulting in many more copies of the protein in the cell. Because these proteins are typically so vital to growth processes--they are often associated with growth factor receptors, or are transcription factors which activate whole sets of other genes--they can cause serious problems if not tightly regulated.
Luckily, one oncogene is almost never enough to cause a tumor by itself. Since mutation is always a risk, whether through errors in DNA copying, or the effects of environmental mutagens such as UV rays, there are many failsafe mechanisms in place to kill off out-of-control cells.