There are many cancers that occur at the same rate across populations, these cancers occur due to random errors during normal biological processes. Other cancers are found at different rates among different populations. These cancers are likely due to lifestyle or genetic differences among the populations. Both lifestyle and genetic contributions to cancer formation are extremely important.
If a mutation is made in a sex cell (egg or sperm) the mutation will be passed on to future generations. These types of mutations are considered germline mutations. In general, germline mutations affect the next generation not the current generation. The mutation that is passed down will be in every copy of the genome in the progeny, as every cell in the body is derived from the original sex cell.
Unlike germline mutations, somatic mutations occur in somatic cells (any cell except egg or sperm) and therefore the effect of the mutation will be contained to the person with the mutation and will not affect their progeny. The mutation will also only be present in cells that are derived from the cell with the mutation; it will not be widespread as it is with a germline mutation.
Image courtesy of Wikimedia