What is the difference between Pre-Qualified and Pre-Approved?

Pre-Qualified:
The Buyer has provided verbal and sometimes hard copy information regarding their income, expenses, assets and liabilities.  Their credit report may also have been obtained.  A Pre-Qualified buyer, at best, can provide the seller with a letter stating an opinion of what they can afford. This has no real value to a prospective seller.

Pre-Approved:
The Buyer has provided written evidence of income, expenses, assets, liabilities and credit.  All information has been verified.  As a result, much of the paperwork for the buyer's loan has been completed.  The Buyer is able to furnish a pre-approval letter (from the lender) to the seller and Listing Broker stating that they are financially capable of purchasing a property.  This buyer will be able to engage in a negotiation that would be taken seriously.