Another way to explain the cells is by how divided they are. Differentiation implies:

How well tumour cells are formed

How cancer cells in tumour tissue are structured

Since structures of cells and tissues are very similar to normal tissues the tumor is called well differentiated. Such tumors appear to develop gradually and propagate.

The cells appear very irregular on a poorly differentiated or undifferentiated tumour, and are not organized in the usual manner. While the usual tissue patterns and structures are absent. Such tumors may be more likely to spread to other areas of the body or adjacent tissues.