What is A Web Crawler?

Web crawlers, also known as spiders, are integral for search engines’ user experience. Without them, search engines, as we know them, would not exist. However, while web crawlers are popularly associated with search engines, they are also used in other use cases, such as online content aggregator sites.

Essentially, spiders are software programs that automatically discover websites. But there is more to their functionality, leading us to the question, what is a web crawler?

What is a web crawler?

Every website whose link you click on a search engine results page (SERP) or online aggregator site is the product of the invisible work done by crawlers. Ideally, and as stated above, these crawlers or spiders discover websites and web pages. They do this thoroughly and sophisticatedly by following hyperlinks included in web pages. Usually, websites contain links to aid in navigation – these hyperlinks direct users or bots to content that is part of the website or an external website.

How does a web crawler work?

Web crawlers use hyperlinks to discover web pages. They simply begin with an array of known websites (URLs) from previous crawls or web addresses provided by site owners. Next, the spiders visit the sites and use the links included on the known web pages to discover new pages, either within the website or external sites. (southside815.com) They repeat this process over and over but not after doing one integral thing.

When the crawlers discover a new page, they go through the content from the first line of the code file to the last. They collect this information, organize it by associating a URL to this data, and store/archive it in databases known as indexes. For this reason, web crawling is also referred to as indexing, as it entails storing discovered pages and their content in indexes.

Upon organizing this data for one web page, the crawlers proceed to the next web page(s) by following the link(s) therein. They repeat this process over and over. Notably, web spiders discover billions of new web pages thanks to this automated but repetitive process. And to ensure the indexes are up to date, the crawlers periodically repeat the entire web crawling process to discover newly created web pages or recently updated content.

What is a web crawler used for?

A spider accomplishes the following tasks:

  • It discovers new web pages and their associated addresses/URLs
  • A web crawler renders the web page, sifts through the content stored in each web page, and collects key data such as all the words, URL, meta description, how recently the site was updated, and more
  • The spider organizes and stores each web page’s key data in an index to enable the search engine or online aggregator to retrieve this data later, presenting it on the SERP based on relevance

Notably, by collecting key data such as words, the index can identify the words that will help search engine users find the web pages. These words, known as keywords, are integral to search engine optimization (SEO) strategies.

While web crawlers collect data from websites, their functionality should not be confused with web scrapers’.

What is a web scraper?

A web scraper is a bot that gathers specific data from websites in what is known as web scraping or web data harvesting. Web scraping is a step-by-step process that begins with requests.

A web scraper sends out requests to the specific sites from which the data is to be extracted. The respective web servers respond by sending an HTML code file containing all the data for the web page(s). Next, the scraper parses through the data, subsequently converting it from an unstructured format to a structured form that humans can understand. Lastly, the web scraping tool avails the structured data for download as a CSV, spreadsheet, or JSON file.

Differences between a web crawler and web scraper

  Web crawler Web scraper
  It is used for large-scale applications It is used for both large-scale and small-scale applications.
  A web crawler collects an indiscriminate amount of data that includes all the words contained in a web page, the URL, meta description, and more A web scraper only collects specific, pre-defined, and tangible data
  The data collected by a web crawler is stored in indexes and is not available for download by humans The data collected by a web scraper is available for download by humans
  A web crawler never relies on the services of a web scraper A web scraper may sometimes depend on a web crawler’s operation
  A web crawler’s output is a list of URLs ranked based on relevance and displayed on SERPs or aggregator sites A web scraper’s output is a downloadable file containing a table with tens of fields and entries



A web crawler is an integral part of the current internet age. It is central to the search engines as we know them. However, while this program collects data from web pages, it should not be confused with a web scraper, which gathers specific information from a small group of websites.

If you are interested to find out more, you can read the article here for an in-depth outlook on web crawlers.

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