A web crawler, also known as a web spider or web robot, is a program that automatically navigates 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 information is then indexed to help search engines like Google provide relevant results to user queries. Web crawlers play a crucial role in maintaining the accuracy and freshness of search engine indexes. By regularly revisiting websites, they ensure that the information available is up-to-date. This process helps users find the most relevant content when they search for specific topics online.