Artboards represent the regions that can contain printable artwork. You resize and set the orientation for your artwork by choosing settings in the Artboard Options dialog box. (In Illustrator CS3 and earlier, you use the Document Setup dialog box to change the document size and orientation.)
You can use artboards as crop areas for printing or placement purposes—they work the same way as crop areas work in Illustrator CS3. Multiple artboards are useful for creating a variety of things such as multiple page PDFs, printed pages with different sizes or different elements, independent elements for websites, video storyboards, or individual items for animation in Adobe Flash or After Effects.
Note: If you created crop areas in an Illustrator CS3 document, the crop areas will be converted to artboards in CS5. You may be prompted to specify how you want the crop areas to convert.