Mortgage Pre-Qualification vs. Approval ?

[Scott Anthony]

I recently had an offer accepted on a residential house for my family and I and as a first-time home buyer, I had a question regarding the mortgage process.  We put in offers on a lot of houses, in which case our mortgage lender attached a "Pre-Qualification" letter indicating that we could afford the home.  We never had to supply any physical information, but we did disclose our debts, salaries, bank accounts, etc.  Now we are gathering a ton of documents and info to give to our mortgage lender.  My question is what is the difference, if any, between pre-qualification and the actual approval process?  Are they just verifying that all of the numbers we told them were correct?  Is there anything hidden that I should anticipate or expect?  Also, when they ask for bank statements, what things do they look for?  Where I spend my money? lol