A loan commitment letter is usually issued once your lender verifies your information and financial statements.
It is issued after you have signed a contract to purchase a property, and is usually contingent on the property appraising for at least the purchase price. A loan commitment ALWAYS specifies the loan amount, terms interest rate, type of loan, and if an adjustable, what your payments will be until the interest rate changes, and the index and margin above that the change will be based on.
A loan Pre-approval does not require you be " under contract", and usually states an approx amount of loan you should qualify for, based upon your credit, income and expenses. These are usually submitted with your offer to purchase, to demonstrate to the seller that you are serious and qualified.