Pre-qualified is essentially a worthless piece of paper. The lender speaks to you on the phone, runs you through the automated approval system, may check one credit score and your done. They do not verify income or assets, nor do they verify employment.
In a pre-approval all of the above are done as a full tri-merged credit report is pulled. This requires you to submit a minimum of two years of tax returns, 2-3 months of bank statements, and to fill out a the lenders initial questionnaire. The best Realtors want this done in advance and it's to the buyer's advantage to be fully pre-approved as it pts them both in a fast track position in terms of when they make an offer as well as the strongest possible position they can be in (short of being cash buyers) when they tender an offer.