When a user is looking at a "product" on any eCommerce site it is possible to both cross sell and up sell other products relating to the product on display.
This is a secondary action and usually appears underneath the main product.
An example would be showing different types or colours of shirts on a shirt product page.
Upselling differs slightly as it describes the ability to buy an additional product relating to the product on show. For example it is possible to upsell guitar strings and music books on a guitar product page.