A crawler, also known as a web crawler or spider, is a computer program designed to systematically browse and index content on the internet. It works by following links from one webpage to another, collecting information and storing it in a database for later retrieval. This process is commonly used by search engines to gather data and create indexes of websites, making it easier for users to find relevant information. Crawler programs are also used for web scraping, data mining, and website monitoring. While they are incredibly efficient at their task, crawlers can also put a strain on website servers and may be blocked by website owners to prevent excessive traffic.