Tumours can be benign or malignant. The benign ones aren’t cancerous. The malignant ones are.
Benign tumours usually grow more slowly and are less likely to come back if they’ve been completely removed. They very rarely spread to other parts of the body, but if they haven’t been completely removed they can sometimes start growing again after treatment.
Malignant tumours usually grow faster. They are different to benign tumours, as they can sometimes spread to other parts of the body, and may come back after treatment.
And some cancers, like leukaemia, don’t form tumours at all, but affect the bone marrow (the spongy stuff inside your bones that produces blood cells) or the blood.