Pre-Qualified - You can be pre-qualified by a lender, an agent and
even yourself. The term means that someone has taken a general look at your income and expenses and plugged them in to a
debt-to-income ratio formula. Loan pre-qualification may not
include an analysis of your credit report or an in-depth look
at your potential to buy a home.
Bottom Line: Pre-qualifying yourself before you start looking
for a home will give you a general idea of the price range you
can afford. It will not nail-down an interest rate for you,
and that factor and others will affect the monthly payments a
lender will allow you to carry.

Pre-Approval - When you are pre-approved, it means a lender has
looked closely at both your credit report and your income. The lender
will tell you the maximum amount of loan they will offer, and
which loan programs you qualify for. You'll also have a better
idea about your interest rate, or you might lock-in a specific
rate.
Bottom Line: Now you can go shopping for a home with
confidence about your buying power, but it still doesn't mean
the bank will approve the loan. Your income and credit report
will be checked again before closing, and the home itself must
be approved.

Loan Commitment - The bank will not issue a loan commitment until it has approved both the house and you. The home appraisal must meet the lender's guidelines, usually meaning the home must appraise at or higher than the sales price. The bank may require more information if the appraiser mentions anything the bank feels should be checked.
Other things that affect a loan commitment:
The home's title must be cloud-free, meaning there are no
problems associated with it (no outstanding liens that can't
be paid at closing; no right-of-way problems; no litigation in progress, etc.).
Your credit profile will be checked again to make sure it hasn't changed in a negative way.

Bottom Line: The loan commitment letter is issued only when
the bank is sure it will lend, so the commitment date on your
contract to purchase should normally be closer to closing than
to the date you make the offer. The home seller can demand to
see that letter as soon as the date has passed, so question
anyone who tries to insert an early commitment date in your
contract.
In some regions, pre-approval means loan commitment. So, ask
your lender to explain all of the terms associated with their
approval process.