A spider, also called a web crawler or bot, is an automated program used by search engines to systematically browse the internet and discover web pages. Spiders visit web pages, read their content, and follow the links on those pages to find other pages — essentially "crawling" across the web like a spider across its web. The content they collect is sent back to the search engine for processing and inclusion in the search index. Google's primary spider is called Googlebot; Bing uses Bingbot; and there are many other crawlers used by various tools and services.

How Spiders Work

A spider starts with a list of seed URLs and begins requesting the content of those pages from the web server. It parses the HTML, extracts content and links, and adds newly discovered URLs to a queue for future crawling. This process repeats continuously across billions of pages. Spiders identify themselves using a "user-agent" string, which is how robots.txt files recognize and give instructions to specific crawlers. Googlebot, for example, crawls in waves — some pages are crawled frequently (popular pages updated often) while others may only be recrawled every few weeks or months depending on their perceived importance and update frequency.

Crawl budget: Google allocates a limited crawl budget to each site — the number of pages Googlebot will crawl within a given timeframe. Large sites with many pages need to prioritize which pages are most important for crawling through internal linking, sitemaps, and robots.txt directives.

Why It Matters for SEO

Understanding how spiders work is fundamental to technical SEO:

  • Pages that can't be crawled can't be indexed — and unindexed pages can't rank
  • Blocking spiders via robots.txt from important pages is a common technical SEO mistake
  • JavaScript-heavy pages can be problematic for crawlers that don't fully render JS
  • Internal linking helps spiders discover deep pages that might otherwise be missed
  • Crawl stats in Google Search Console show how Googlebot is crawling your site