Difference Between Sales Price and List Price in Salesforce

Products, opportunity products, and price book entries all work together.
 
Products are the items you sell. These are generally the place where pieces of information like the name of the product, product code, or product description are contained. 
 
Opportunity products are the link between opportunities and products. You choose from items in your products to add to an opportunity, and that instance of that product on that opportunity is an opportunity product. Here is where users put values like quantity of items purchased, your start and end dates for these products related to this opportunity, and the price you are selling this product for on this opportunity.

Price book entries store the price of any given product in a given price book. Price books are used to store different sets of prices for the same product. Think retail pricing versus wholesale pricing. If you do not have this complexity you will only have one price book (the standard price book) and one set of price book entries.

When you add a product to an opportunity it automatically uses the price book you have attached to the opportunity to grab the price book entry for that product. You can restrict permissions so that salespeople cannot edit these prices for products as they are added to the opportunity.

The list price and the sales price don't always match.  The list price will be the value in the price book and the sales price is what the sales rep really sold the product for. 
 

Example:

Let’s use as an example your standard price book. This is your master price list, your base price, from which all other price books will be derived. 
 
You have a standard price for the product "mangoes" to be $3.50. When you create your different price books (Retail, Wholesale, Latin America, EMEA, APAC) you can now modify that price for these different price books. You can increase the price in EMEA to $4 but lower the price for APAC to $2.50.

The list price when you choose the EMEA price book will be $4. The sales price is what the sales rep really sold this product for.  So if they gave a discount of 25 cents, the list price of $4 becomes a sales price of $3.75. This is one form of discounting; the price could also be raised.

Assuming the permission is turned on in the user's profile (“Edit Opportunity Product Sales Price"), sales price can be edited by the rep. In other words, the rep can control the sales price if the permission is turned on for the user.