Remember the Rous Sarcoma virus that caused chickens to get cancer. The virus carried a mutated version of a gene normally present in chicken cells. Over activity of this gene lead to cancer. Infection with the virus lead to overactivity and the development of cancer. Other oncogenes are now know. They usually act in a dominant fashion in that if there is another normal allele present in the cell then the phenotype is still malignant.
Tumor suppressors are genes whose activity normally inhibits tumorigenesis. When this activity is lost tumorigenesis occurs. The loss of the effect of tumor suppresors usually occurs in in recessive fashion. Loss of the function of both alleles is usually required to induce tumorigenesis. Examples of tumor supressor genes include, Rb (retinoblastoma), APC (Polyposis coli) and p53 (Li-fraumeni syndrome).