The normal distribution is that the bottom layer is made of round young cells. As the cells mature they rise to the surface and flatten out, so that on the surface the cells are flat. In cervical dysplasia there is a lack of this organized growth process. If this condition is not treated, it often will grow into an invasive cervical cancer. In invasive cancer the cells are not only disordered throughout the entire thickness of the lining, but they invade the tissue underlying the surface.
Invasive cancer is treated entirely differently than dysplasia.
It is important to realize that the different grades of cervical dysplasia are not a stepwise progression but rather represent a description ranging from a slight irregularity in the cells to a full thickness abnormality. Because of this confusion a new classification system for cervical dysplasia was developed. Again, you should remember that cervical dysplasia should be considered to be pre-cancerous, but not cancer. Statistically, mild dysplasia is more likely to go away without any treatment that severe dysplasia, which has a higher probability of becoming invasive cancer over time if not treated. This means some cells that are slightly funny looking, but not abnormal enough to call dysplasia.