Some cancer cells pick up mutations in their DNA which means that they break down to environment around them. Tumours are surrounded by a mesh of proteins that keeps them contained. Some cancer cells can break this down or they can squeeze through gaps in it by changing their shape. This means that these cancer cells can get into blood vessels and spread to other organs in the body.
This is all about how sticky cells are and whether they can escape from the tumour they were born in. Some cancer cells will invade the blood vessels and can then be carried round the body to act as a seed for a new tumour elsewhere.
Cells need to break away from their tumour and they can do this by breaking down the local environment so they have an escape route
That depends on whether the tumour is bening (good) or malignant (bad). Bening tumours can be treated, normally removed with surgery, so they stop growing and spreading to other areas of the body. Malignant tumours, however, could spread because their cells are more ‘clever’ and can escape from it and travel through our blood system, finding a new place to start a new tumour. That is called metastasis, and unfortunately it’s very hard to cure because cells could always escape and travel to other parts of our organism :-/