A web crawler, also called a spider or robot, is a programme or script that automatically navigates through the pages of a website, following the links from one page to the next. The purpose of a web crawler is to collect information about the structure, content and links of a website, which is then used for various purposes, such as indexing a website for search engines, monitoring the website for changes and analysing website data.
When a web crawler visits a website, it first visits the home page of the website and then follows the links on that page to other pages within the website. As the crawler visits each page, it collects information about the page, such as the text of the content, the title and the URLs of any links on the page. The crawler also records the URLs of images, videos or other types of media on the page.
Web crawlers can be customised to perform specific tasks. For example, a search engine web crawler focuses on indexing website content, while a monitoring web crawler focuses on detecting changes to website content.
The way a web crawler works is usually based on an algorithm that regulates how many pages are visited per second, how deep it goes into the website and how it follows links. Rules that ensure that the web crawler skips certain types of pages, such as those with certain file extensions or those located in certain directories, are also common.
Web crawlers are an essential part of how search engines work. They are responsible for discovering new websites and adding them to the search engine's index. They also help search engines understand the structure and organisation of a website, which can affect its ranking in search results.
Web crawlers can also be used to monitor a website for changes and analyse website data, such as traffic patterns, user behaviour and more. This information can be used to improve website design, marketing strategies and the overall user experience.
It is therefore important to pay attention to how often and how many pages are crawled by a web crawler in order to avoid negative effects on the website.