First, what is an "extended warranty"? The insurer, most often not the retailer you are buying it from, promises to repair the item you are buying, or replace it if it needs repair or replacing for a specified amount of time.
These warranties are not for the life of the product, but over a period of two to five years. Some warranties begin immediately, others at the end of the warranty offered by the manufacturer.
Some retailers offer a replacement plan, promising a replacement or even store credit should the item you purchased fail during the plan's duration. Usually, this offer is made on lower cost electronics or merchandise.