These documents are similar and are often used interchangeably. While they vary a bit from each other, they are used for the same purpose which is to show intent and an agreement to agree. Most are considered "nonbinding" unless the wording is included to make it binding.
Use a Letter of Intent if: You want to take serious steps towards negotiating the purchase of all or part of a company. You want to secure your interest in a company and request that the sellers exclude negotiating with another buyer.