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.
Beware of an individual or group of people who call themselves Investment Bankers or a Buying Group. When they cannot provide you with personal financial statements or a financial statement from their company, you have a very slim chance of ever closing the transaction. The seller should have a copy of the financial statement from the buyer attached to all offers.