Search form or sitemap?
Finding specific content on a complex website is hard. Users spend much time on it, while designers try to facilitate their experience—through clear hierarchical navigation, breadcrumbs, headings and other techniques. But sites having thousands of pages need a more direct approach.
Search forms must be evident and instantly useful, so that the user feels invited to use them. Some web designers place them around meaningless content, which makes them less visible, usable and context-appropriate.
Search forms support users in quickly finding relevant search results, depending on how good their search algorithm is. The same content would be otherwise hardly accessible—with the user having to browse through many pages and possibly losing hours. A drawback is that search results might be less relevant if millions data entries compete for the same keywords—the complexity of search queries grows with the amount of content to sieve through. It's also slower for users to fill forms than to click on links.
Sitemaps provide lists of keywords that lead users to the most important information on a website—usually in a meaningfully organized and accessible manner. They usually show the site's hierarchy of pages, which can be complex on its own. The user may need to traverse all options in it to find something of interest. An advantage is the discovery of a more appropriate information.
Search forms and sitemaps can be used both to complement each other—with the former placed on the top and the latter on the bottom of the page. The better a search engine is, the less a sitemap is needed.