Most of the page content comes from the database connected to the Web site. Visitors love them since they get quick access to the information they want. Search engine spiders have a much tougher time with dynamic sites. Some get stuck because they can't supply the information the site needs to generate the page. Other spiders deliberately stay away from dynamic pages to avoid getting trapped in the site.
Visitors find information in a dynamic site by using a search query.
But a search engine spider doesn't know to use your search function - or what questions to ask. Spiders usually stop indexing a dynamic site because they can't answer the question. Then the spider and your server navigate a never-ending loop where a request for a page is met with a request for information. Getting a spider trapped inside your server is not just bad for the spider. The repeated requests for pages can crash the server.