By default, Drupal uses and generates URLs for your site's pages that look like "http://www.example.com/?q=node/83." This style of URLs can be hard to read, and can prevent some search engines from indexing all the pages of your site. Research suggests this may not be as big of a problem for major search engines as it once was; however, it is worth noting the recommendation from Google's webmaster guidelines stating:
If you decide to use dynamic pages (i.e., the URL contains a "?" character), be aware that not every search engine spider crawls dynamic pages as well as static pages. It helps to keep the parameters short and the number of them few.