A web crawler is an automated program that systematically browses the internet to collect information from websites. It starts with a list of URLs and visits each one, extracting data such as text, images, and links. This process helps search engines like Google index web pages, making it easier for users to find relevant information. Web crawlers operate by following hyperlinks on web pages, allowing them to discover new content. They play a crucial role in maintaining up-to-date search results and can also be used for data mining, research, and monitoring changes on websites.