How is the cell cycle linked to cancer?

Well im not a scientist or anything but i know its something to do with the uncontrolled growth of cells. its like... your bodys cells make copies of themselves. that's the reason you sometimes get a tumor...is because there are cells where they shouldn't be growing things that shouldn't be there. Some cancers.. like leukemia usually don't form tumors these cancer cells involve the blood and blood-forming organs and circulate through other tissues where they grow. maybe something you don't want to know but we all have cancerous cells inside of us. our body recognizes them and fights them off though...usually