A crawler is an automated software that searches all pages on the internet. A crawler checks every line of content on a website and follows every dofollow hyperlink to another page or website. A crawler is like a bot that automatically searches a long list of URLs. Every search engine has its own crawler, for instance Google has the Googlebot.
Crawlers are used to index both new and existing pages, so that these pages can be found in the search results. Therefore, having a clear internal link structure and a sitemap is of great importance in search engine optimisation.
The terms crawler and spider are often used interchangeably. However, there is a difference in both terms. A crawler is used to search entire search engines, whereas a spider is used to search a website.