Hi Ethsey. Welcome to the forum. I am not a doctor.

Cancer isn't a single disease, but rather it's a general name for a whole range of similar diseases.

It usually starts with a single cell mutation. That cell, instead of working as part of the team goes free-lance. It no longer responds to control signals from the cells around it. It starts multiplying to form a lump, or cancerous tumour.  Over time, the cells accumulate more mutations which makes it more aggressive.  After more time, it starts to split and chunks of cells circulate around the body where they lodge in random locations, and form more cancerous tumours. By this point, the cancer is probably incurable and terminal.

Intervention in the early stages can often effect a cure. Sometimes the body's own immune system will kill off mutating cells. If that doesn't happen, the patient may experience symptoms which get reported to the doctor, and the doctor in turn refers the patient to a hospital.  If the cancer hasn't advanced too far, then it is probably curable (although the means of the cure varies between different cancer types).  But if the patient doesn't experience any symptoms, or maybe ignores some symptoms then by the time he/she arrives at a hospital, it may already be too late. 

So, in general, a cancer if caught early enough is usually curable. It takes time to become terminal, probably a period of many months.  This is why it's important to be aware of "red flag" symptoms and to act swiftly if you experience any of them.