A pre-qualification means that you have answered a couple preliminary questions about your 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 you have spoken at greater length with a lender, and have 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, you finding a property.
Sellers take pre-approvals much more seriously than a basic pre-qualification, with good reason. They want to be sure they are taking their property off the market for someone who is almost certain to be able to acquire a loan.