Also: spider or bone
A crawler is a software programme that systematically searches the web to index web pages for search engines as Google. These crawlers play a vital role in the way search engines work and how websites are ranked in the organic results.
A crawler usually starts with a list of web addresses from previous crawls and sitemaps provided by website owners. When the crawler visits a page, it looks for links on that page and adds them to its list of pages to be crawled. This process continues until the bot has indexed a certain number of pages or reached some other limit. It is crucial for website owners to have a clear and easily navigable internal link structure so that crawlers can go through the site efficiently and index all important content.
It is also possible to give instructions to crawlers on which pages should or should not be crawled using a robots.txt file. This can be useful if certain parts of a website are private or do not add value for search engines. However, it is important to note that not all crawlers respect these instructions. Therefore, it is always good to take additional measures, such as setting no follow tags or password protection, to protect sensitive information.