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In this case the set of sections that come before the body of the book are known as the front matter.
When the book is divided into numbered chapters, by convention the introduction and any other front-matter sections are unnumbered.
Keeping the concept of the introduction the same, different documents have different styles to introduce the written text.
If a Userguide is written, the introduction is about the product. In a report, the introduction gives a summary about the report contents.