Your Homepage is the index of your website. It is not only the main page - it's also the introduction, index, and default search engine listing for your entire website. Your homepage should have links to all of the important pages on your site.
Your contact page is your sale closer. The main goal of most websites is to eventually lead visitors to the contact page so your marketing department can close sales with "hot-calls" (leads that are generated by the contact page).
The about page describes your company (i.e. partners, employees, history, mission statement). This page is important because it establishes you as a trusted and legitimate company.
Sitemaps have been going crazy this year because it is the easiest way to layout a map of your entire website for search engine spiders to follow. Most of the major search engines have also adopted standards and support for XML sitemaps.