In Saleforce.com, the Opportunity, Product and Pricebook are standard objects. How are they related? In actual fact, an opportunity has one or more products and vice versa, a product can be prospected in one or more opportunities.
The approval process cannot directly trigger the apex trigger. You will need to create a field (e.g. checkbox) that is checked by the approval step in the approval process. Then start a trigger based on that field.