A configurable product is a product where the customer has the ability  to select certain attributes, like size and color. In regard to the catalog structure it is one product with individual products underneath it, that the customer actually buys. For instance, a customer does not buy a T-Shirt, but rather a Blue T-shirt, or a Small T-shirt. The T-shirt is the configurable product, with the additional products of a blue T-shirt or small T-shirt, underneath this umbrella product of T-shirt.