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.
Here are some other terms that may also be included: Confidentiality » the agreement and any information learned will remain confidential. Covenants » things each party must do while negotiations are taking place. Special Terms » any special terms that the parties agree will be in the purchase agreement, such as leaving certain items of furniture or hiring certain employees.