If you’re prequalified it means that you POTENTIALLY could get a loan for the amount stated to you, assuming that all of the information you provide to the bank is accurate and true. This is not as strong as a preapproval.
If you’re preapproved, it means that you have undergone the extensive financial background check, which includes looking at your credit history, previous tax returns and verifying your employment – and the lender is willing to give you a loan, basically meaning you’re approved!
You will usually be provided an accurate figure which shows the maximum amount that you are approved for.  Most sellers prefer buyers that have been preapproved because they know that there will not be any problems with the purchase of their home.