There are different ways to classify testcases.
But it could be one of them 
1. Classify testcasees  which follows normal business flow
2. Classify testcases which checks Link navigation (this is 
more for Web based applications)
3.Field validations (Data which we are enterering is 
following business rules
4.Content verification :Content dispalyed on the 
application are correct 
5. Negative testcases - Flows other than normal business 
flows

This classification for functional testing