The difference is a Letter of Intent is basically a letter stating the kind of offer you are contemplating in making. Usually it contains what I call the GENERAL items of price, terms and closing date. Most of the other legal STUFF is left out.
Always construct the letter of intent with honest and detailed facts. Nothing must be overlooked as it can cause problems in the future. Be prepared to negotiate terms, and always know what you are willing to accept if you should get a counter offer.