What are oncogenes and how do they relate to tumor formation?

Definition: an oncogenes is a gene, whose altered expression, over-expression, amplification and the presence of mutation lead to tumors arising.These most often are part of intracellular signalling pathways, that regulate the cell cycle and cell death, mostly by inducing progression of the cell cycle and inhibiting apoptotic signals. So if they become more active(due to a mutation or amplification), then the cell will be able to divide rapidly, overriding checkpoints, thus continuing division even if it should not (eg. DNA is damaged), or it evades apoptosis (programmed cell death, induced by internal or external signals). Thus, when expression of certain oncogenes increases, a cell will be able multiply indefinitely and/or evade destruction, which will lead to the formation of cancer, as it will become independent of outside stimuli and internal checks (depending on the oncogene), thus it will undergo rapid, uncontrolled division, that will form a tumor.