What are the differences between normal cells and cancer cells?

Cancer cells are not foreign cells, but abnormally functioning body cells, and may therefore escape immune detection, which is one of the reasons why they are so difficult to treat. 

Here are some differences between normal cells and cancer cells:

1.) Shape 
Normal cells: small nuclei, large cytoplasmic volume, cell size and shape is usually similar among cells of the same type
Cancer cells: large nuclei, small cytoplasmic volume, cell size and shape varies (even among cells of the same type)

2.) Arrangement
Normal cells: arranged into discrete tissues
Cancer cells: disorganised arrangement

3.) Functions
Normal cells: differentiated cell structures (specialisation - carry out very specific functions according to cell type)
Cancer cells: loss of specialised features, elevation of certain markers (that may lead to increased division and proliferation - and therefore to the uncontrollable growth of the tumour - an example is BRCA in breast cancer)

4.) Growth and movement in space
Normal cells: low levels of dividing cells, stay within clear boundaries of cell tissues
Cancer cells: large number of dividing cells, tumour boundaries are absent or poorly defined