PricebookEntry object is a product entry in a pricebook. This object allows products to be linked to standard price book or custom price book. One price book entry is linked to only one product. One price book entry can only appear in one pricebook
Pricebook are list of products and their associated prices. Each product and its price is called a pricebookentry. So, if you want to speak in relationship terms, a Product may be on multiple Pricebooks; this is managed with a junction record called PricebookEntry.