A pre-qualification means that the buyer has answered a couple preliminary questions about their income and expenses, and based on that information should be able to qualify for a loan in a certain price range.
A pre-approval is verification that the buyer has spoken at greater length with a lender, and has already submitted a basic loan application with detailed income and expense information. Instead of an estimation of affordability, a pre-approval is actually a loan approval contingent upon final verification of documents and of course, the buyer finding a property.