These two terms are used interchangeably by most people, but there are some key differences between them.
'Pre-Qualified' means that you have given me all of the information I need to reasonably estimate what your maximum purchase price is. It may or may not involve a full credit check - that's completely your choice! It doesn't include a rate hold feature, so if rates go up while you're shopping for a home, you'll end up with that higher rate amount.
'Pre-Approved' means that you have given me all of the information that both me and the mortgage lender need to reasonably estimate what your maximum purchase price is. It requires a full credit check. It allows you to put a hold on current rates if we're in a rate environment where they might go up! While a pre-approval doesn't guarantee an approval, they can be handy if you're in an increasing rate environment or if your situation is unique and you need that extra assurance that the mortgage lender is okay with giving you financing. Sometimes, a pre-approval certificate can give you a competitive edge when you're putting offers in on homes because it gives the seller more confidence that you can actually afford to buy their house.