Cannabanoids have been proven to cause death in cancer cells. They inhibit the growth of multiplication of cancer cells, thereby reducing the growth of tumors.
There are several ways in which this is made possible. Anti-angiogenic effects prevent formation of new blood vessels needed by the tumor to grow. Anti Metastatic effects prevent cancer from spreading to other organs. The cannabanoids block the ability of the cancer cells to spread.
This video offers even more sound scientific explanation as to exactly how cannabis cures cancer.