A web crawler is an automated program that explores the internet in a methodical way. You might also hear them called spiders, bots, or web robots. Their main job is to visit websites, read their pages, and gather information to create a massive index of the web.

Just as a librarian follows references from one book to another, a crawler follows links from one webpage to the next. It starts with a list of known pages and then “crawls” through the hyperlinks on those pages to discover new ones. This process runs continuously to keep up with the ever-changing content online.

The most common use for web crawlers is to power search engines like Google, Bing, and DuckDuckGo. Without crawlers, these search engines would have no way of knowing what pages exist on the web. When you type a search query, you're not searching the live internet in real-time. Instead, you're searching the massive index that web crawlers have built and organized.

But search engines aren't the only ones using crawlers. Here are a few other examples:

• Price Comparison: E-commerce sites use crawlers to check the prices of products on competitors' websites.
• Data Analysis: Researchers and data scientists crawl websites to gather large amounts of text and data for analysis, like tracking public opinion on social media.
• Archiving: Services like the Internet Archive use crawlers to save snapshots of websites, preserving a historical record of the web.

Web crawlers are the invisible workers that make the modern internet searchable and interconnected. By systematically cataloging web pages, they build the foundation for finding information quickly and efficiently.

Ready to check your understanding?

Now that you understand what crawlers are and why they're important, you're ready to learn about the different components that make them work.