Search engines use software called spiders or crawlers to crawl websites, follow links, and record only text content. This text is then indexed according to keywords and stored in a database. When a search is performed, results are returned based on a relevance calculation between the search terms and indexed keywords. Keywords should be specific, included in titles, headings, links, and content to increase relevance. Backlinks from other relevant sites also improve relevance through the anchor text used. Images and other non-text content should include descriptive alt text for indexing.