For: Red Dwarf characters#The Skutters

A web crawler (also known as a web spider, web robot, or—especially in the FOAF community—web scutter) is a program or automated script which browses the World Wide Web in a methodical, automated manner. Other less frequently used names for web crawlers are ants, automatic indexers, bots, and worms.

This process is called web crawling or spidering. Many sites, in particular search engines, use spidering as a means of providing up-to-date data. Web crawlers are mainly used to create a copy of all the visited pages for later processing by a search engine that will index the downloaded pages to provide fast searches. Crawlers can also be used for automating maintenance tasks on a website, such as checking links or validating HTML code. Also, crawlers can be used to gather specific types of information from Web pages, such as harvesting e-mail addresses (usually for spam).

A web crawler is one type of bot, or software agent. In general, it starts with a list of URLs to visit, called the seeds. As the crawler visits these URLs, it identifies all the hyperlinks in the page and adds them to the list of URLs to visit, called the crawl frontier. URLs from the frontier are recursively visited according to a set of policies.

Crawling policies

There are three important characteristics of the Web that make crawling it very difficult:

• its large volume,
• its fast rate of change, and
• dynamic page generation,

which combine to produce a wide variety of possible crawlable URLs.

The large volume implies that the crawler can only download a fraction of the web pages within a given time, so it needs to prioritize its downloads. The high rate of change implies that by the time the crawler is downloading the last pages from a site, it is very likely that new pages have been added to the site, or that pages have already been updated or even deleted.

The recent increase in the number of pages being generated by server-side scripting languages has also created difficulty in that endless combinations of HTTP GET parameters exist, only a small selection of which will actually return unique content. For example, a simple online photo gallery may offer three options to users, as specified through HTTP GET parameters. If there exist four ways to sort images, three choices of thumbnail size, two file formats, and an option to disable user-provided contents, then that same set of content can be accessed with forty-eight different URLs, all of which will be present on the site. This mathematical combination creates a problem for crawlers, as they must sort through endless combinations of relatively minor scripted changes in order to retrieve unique content.