A map book is like an atlas: it takes a single map and divides it into "tiles" by placing a grid over it. Each square of the grid becomes a map tile. Each tile is then rendered on a separate page, which you can publish to a plotter or to an electronic format called DWF. Once you publish a map to DWF, you can share it with people who do not have a copy of AutoCAD Map 3D toolset. These people can download Autodesk Design Review, a free viewer available from http://www.autodesk.com, to see the maps.
Map books use viewports to organize the information on each page. A viewport is like a frame on a web page. You can create a custom viewport as an inset on one or more of the map book pages. Pan and zoom within that viewport to display an enlarged detail of the map.
You can also link a regular “key” viewport on the map book to a copy of the main map file. Then, every tile of the map book displays the overall map in a small window.