The document provides information about writing formal and informal letters. It discusses the differences between formal and informal letters in terms of vocabulary, sentence structure, and tone. Formal letters use more formal vocabulary like "would" instead of contractions and carefully constructed complete sentences. Informal letters have a more casual tone and resemble spoken English with features like contractions, colloquial vocabulary, and sentence fragments. The document also includes examples of rewriting sentences from informal to formal and vice versa to demonstrate these differences.