How Many Kinds of Najasah are There?

It is seen in fiqh books that the things that are not regarded as clean but regarded as najis (dirty) are examined in two parts:

1 – Heavy Najasah (Najasah Ghalizah),

2 – Light Najasah (Najasah Khafifah)...

This differentiation does not depend on whether the dirt is little or much but whether it invalidates the prayer or not. 

Whether the dirt is little or much does not matter; they both pollute things. For instance, if a small amount of najasah, whether ghalizah or khafifah, falls into the water, it makes the water dirty. From then on, making wudu out of that water will not be valid.