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.
State that, no binding contract is to come into effect, except to the extent set out in the letter and that neither the letter nor any work done or payment made under the letter shall be deemed to be a waiver of the requirement to provide a binding contract. Finally, every effort should be made to finalise the contract as soon as reasonably practicable after the letter of intent has been signed.