text: There are specific genetic mutations that cause cancer, but it's different for pretty much every cancer type and every individual with cancer.  That said, there are certain common themes, pathways that are commonly mutated, such as DNA repair pathways, cell cycle checkpoint pathways, cell death pathways.  \nStructurally there are differences between normal and cancer cells, they look different.  They tend to be not differentiated, they tend to invade, they tend to metastasis, etc.  \n\nThere are entire fields of study and textbooks devoted to cancer biology. Here is one text online at the NCBI.