product vs product type vs product display in drupal commerce
Product types is like node types. The main thing about is that you can tell them apart and you can put different fields on them. A product type could be books, but if your books and board games are essential the same (they need the same data) they could be the same product type, where as t-sheets would need to be a different product type, since you probably want to add data for color, size, model which doesn't make any sense for books and games.

A product is an instance of a product type, meaning it could be a green medium sized t-shirt for men, or 'The Definitive Guide to Drupal 7', along with price etc.

Product displays is the main way of displaying your products to the end user, since out of the box products can't be displayed. For this commerce has built the product reference field and the add to cart form and such features to do the trick. Meaning that you can display your products on other entities. Most often you want to use nodes for this, but in theory you can use Profile 2 with the profile entity, to allow users to sell stuff on their profile with a product reference field.

You could also use Views to display products to the end user, this is what I did with my first Commerce site, where Commerce wasn't matured yet. I wouldn't recommend this, as you loose the ability to combine products with the add to cart widget. But if you are selling antique furniture, where every product is unique, this could work out well for you, as you only would need to create products and not product displays as well.