Magento has built-in options that allow you to set up various product relationships. This can help to give choice or increase the basket value of the visiting customer.
Related; If items are related in some way and can be of interest to customers viewing/buying a particular product, you can relate these products to the particular product. When a customer goes to the product details page of the product, the related products will be shown together in one of the sidebars, dependent on the design or extensions in use.
Up-sells allow you to offer products that are similar to the one that the customer is viewing but are better and more expensive. For example, as up-sells to a particular smart phone model you can offer other with a better camera or a larger storage. The products offered as up-sells will be shown on the product details page under the description for the particular product that the customer is viewing.
Cross-sells are not shown on the product details pages but they are displayed in a separate area on the shopping cart page.