Products can be Related to each other in three ways: Up-Sells, Cross-Sells or by having the same tags or categories.
To add an up-sell or cross-sell to a product:
Up-sells are products that you recommend instead of the currently viewed product.
They are typically products that are more profitable or better quality or more expensive. Depending on your theme template, these products display on the single product page underneath the product’s description.
Cross-sells are products that you promote in the cart, based on the current product.
They are typically complementary items. For example, if you are selling a laptop, cross-sells might be a protective case or stickers or a special adapter. Or if you’re selling a ninja t-shirt, they could be a ninja hoodie and ninja socks.
Depending on your theme template, they display on the cart page underneath the cart products table with a thumbnail image.
Related Products is a section on some templates that pulls products from your store that share the same tags or categories as the current product.
These products cannot be specified in the admin, but can be influenced by grouping similar products in the same category or by using the same tags.