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.
Build a list of things that you will be addressing in the Purchase Offer and decide how you want each of them handled. Now you will not be able to cover EVERY scenario but you will be much further ahead (and prepared) when it comes to crunch time. What do you do then? Present the offer to the seller. Make sure you have an ending day and time for your offer to expire. You do NOT want the seller thinking this over for too long.