New Post: Product list category grouped

I mean:
i have Category A with Product 1,Product 2,Product 3
i have Category B with Product 4,Product 5,Product 6,Product 7

I need to make a tree list with category and product like

Category A
|- Product 1
|- Product 2
|- Product 3

Category B
|- Product 4
|- Product 5
|- Product 6
|- Product 7

tell me if I'm clear, in other way I try to exaplain another time
thanks for your reply
Fabio