Introductions to formal reports may be quite lengthy, providing background information, definitions, and assumptions to help the reader develop a perspective for what follows.
Introductions to informal reports are rather brief, often serving as an executive summary (background information, conclusions, and recommendations).
Introductions to letters and memos are very brief and, unlike formal reports, are not separated from the rest of the text by headings. They establish the tone and purpose of the document, setting up the points that follow and often referring to previous conversations, meetings, or correspondence.