When you’re prototyping sites in the early stage, the three main artifacts are: sitemaps, storyboards, and schematics.  In the book, The Design of Sites, by Douglas K. Van Duyne, James A. Landay, and Jason I. Hong describe the three artifacts as follows:

• Sitemaps - a high-level diagram showing the overall structure of a site.  You use it to reflect an understanding of the information structure or architecture of the site as it's being built, and the navigation structure or flow through the entire site, at the macro level.
• Storyboards - a sequence of Web pages depicting how a customer would accomplish a given task.  You can use storyboards to show important interaction sequences or flows through  a site.
• Schematics - represent the layout of the content that will appear on individual pages.  They don't usually include images, instead they have placeholders and labels.