Oncogene is a gene that promotes cancer growth while proto-oncogene is the gene that prevents cancer.
One of the characteristics of cancer is their uncontrollable growth. In cell cycle, there is some checkpoint that will detect genetic damage and repair it, or stop the cell cycle. Proto-oncogene would be responsible for repairing the DNA or signaling the cell to stop dividing. When proto-oncogene damaged, it becomes oncogene and the cell is much more likely develop into cancer.