Lenders typically need only a few data points in order to issue a generic pre-qualification letter. You will need to provide your correct legal name as used on your filed federal tax returns, your date of birth, and social security number. Next you will provide non-verified income and asset information. Once an official mortgage credit report is merged into a lender's origination system, that lender can estimate a payment and provide a letter based on that information. These letters are not very reassuring to a seller as no facts or documents have been reviewed by an underwriter.
A pre-approval letter typically means that an underwriter has reviewed your pay stubs, other income documents as well income documents, and credit facts. This type of letter is significantly more helpful in helping your agent create a strong offer.