Spiders are programs that systematically browse the web for the purpose of web indexing. They are also known as web crawlers or bots and are used by search engines to discover and index web pages, making them searchable. Spiders follow links from one page to another, gathering and cataloging information about the web content they encounter. In the context of data scraping, spiders can be designed to extract specific data from websites as they crawl, often using rules or patterns to identify and capture relevant information. They play a crucial role in both search engine operations and large-scale data extraction projects.