Magento

Related products are products that a customer may want to buy *as well as* the product they are viewing. They will appear by default on the products page, with a tickbox to add them to the cart.

Up sells are products that a customer may want *instead of* the current product, ie a gold plated version etc. By default it will show a link to the upgraded products page on the current products page.

Cross sells are similar to related products in that a customer may buy them *as well as* the current product, but they appear only on the customers cart. There is a button allowing them to add this cross-sell product to their cart as well.

You can find a guide to Managing Products in PDF format at http://ipicdg.com/EC/Magento/ManageProducts.pdf.