A portfolio is a tool to represent your images to people or businesses. You may be showing fine art photography or portrait photography or wedding photography or commercial photography. . . well you get the idea. In many ways it's like a resume: it's a detailed introduction for a person in fine arts or commercial art. Your book is often the most important tool you have, more important than your camera or your lights. You can find more information with this link: siskinphoto.com/blog/?p=2653