In order for search engines to be able to trawl your site effectively, a sitemap is required to point out all of the different areas and pages. The sitemap needs to be in .XML format, which is widely accepted by all major search engines. Failure to have a sitemap will have negative effects on your website’s search engine ranking, as search engines will trawl your site less favourably. You can use an online tool such as http://www.web-site-map.com/ to help you create a sitemap.
If you are using AJAX to pull content into a page dynamically it’s important you also read this Google article https://developers.google.com/webmasters/ajax-crawling/docs/getting-started and apply what’s said to your sitemap.