UP sell products are the products related to the product that get you more profit margin or featured products in same category you need to more. Related products are products that the customer may buy in addition to the current product. For example A necklace can have matching earrings as related products.
Difference between up-sell and related product :- Up-sell : Up-sell products are items that your customer could buy instead of the product that is being viewed. These products may be of a better quality, more expensive, be more popular or produce a higher profit margin. Related product : Related products are meant to be purchased in addition to the item the customer is viewing. Related products are shown in the right column of web store in a separate block.