Pre-qualification is typically based on data you submit and is more of a preliminary estimate of the mortgage size you could qualify for.
Pre-approval is usually the more involved step and signals what a lender can offer you, though it isn't an actual guarantee.
While both have their purposes, pre-approval tends to be more useful for showing sellers you're ready to buy.
A pre-approval is what I call 'show me.' What I mean by that is let me see the real documents that I—or underwriting—will need for an approval:
pay stubs, W-2s, bank statements, credit report, tax returns, etc.,