In Magento, an upsell is a product that you would like your customer to buy, instead of the one that the customer is viewing. For example, suppose a customer is viewing a basic, hand-cranked coffee grinder; an upsell would be a more elaborate, expensive coffee grinder. Magento puts upsells at the bottom of a product page, under the heading You may also be interested in the following product(s).
An upsell should yield a higher profit than the product the shopper is viewing. Matching an upsell to a product can be an art. The upsell needs to be more expensive than the current product, but not so expensive that the shopper immediately rejects the upsell. The upsell needs to offer clear advantages over the current product, without being so different that someone interested in the current product will consider it too different from what he/she wants. Also, the pictures and descriptive text for both products need to offer a direct and compelling comparison. All of these considerations...