Upsells are products that you can recommend instead of the currently viewed product. They are typically more expensive, more profitable or better quality that the the currently viewed product.
Cross Sells are products you can promote in the cart, based on the currently selected product.
To configure this – go to the Product Edit page and scroll down to the linked product section.
Choose which option you wish to configure, and search for a product in the corresponding text field.
When you are done, scroll down and select Save Product