cell biology - What are the differences between cancer and tumour?

What are the differences between cancer and tumour? I mean is it in the DNA or shape or something else... And how can a benign tumour turn into a malignant tumour? The body has a lot of tumours all the time, however not all of them become cancerous, why is this so?

So my main question is: What is so special about cancer cells that they can get out of control unlike an ordinary benign tumour?