The main difference is that personal letters are very informal in grammar and the vocabulary is much simpler and usually friendly, thus very personal in style. A business letter has much higher standards, formal grammatical structure, and standardized vocabulary (to convey exact meanings).Often business people and corporations have example letters indicating the level of formal style approved and in use by the business world. For example, a newspaper would have a Style Book that its writers and editors are expected to follow and comply with.