A sitemap is like a table of contents for your web app or site. It visually summarizes the organization of pages & content on your site. This article will provide a great explanation of site maps.
How to Create Sitemaps
MS Word and Powerpoint can be used to build site maps quickly. These tools are easily accessible (almost everyone has MS Office installed), and nearly anyone can use them. There are other good tools like OmniGraffle, Sketch etc too but they are pricey.