It isn't simply just a "defect cell". Its a defect cell that's lost its ability to stop dividing and does not know when/how to die. Making more of itself with its own same dna which makes the new cells also unable to stop and die. Keep in mind, these cells don't perform their original job properly (if at all) yet they still require nutrition from the body. Basically, taking up more and more resources.

If you're unlucky, this process can spread to other parts of the body (malignant) and damage organs and tissue (will certainly die if left untreated). If you are lucky, these malicious cells can be confined to a particular area (benign) and can more easily be treated if treatment is deemed necessary.

The reason cancer is still so rampant and not 100% curable in all cases is because there are so many different "types" of cancers (over 100 known!) that react differently to different things. The "stage" of the cancer also plays a big role on how likely treatment can be effective which is why its always important to be on the lookout early on.