Spiders -
Web crawler

A Web crawler (also known as a Web spider or Web robot) 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 (Kobayashi and Takeda, 2000). This process is called Web crawling or spidering. Many legitimate 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 Web site, 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. Spiders - Web crawler refers exclusively to practices that are dishonest and mislead search and indexing programs to give a page a ranking it does not deserve. "White hat" techniques for making a website indexable by search engines, without misleading the indexing process, are known as search engine optimization (SEO). SEO techniques do not involve deceit. Search engine spammers, on the other hand, are generally aware that the content that they promote is not very useful or relevant to the ordinary internet surfer. Search engines use a variety of algorithms to determine relevancy ranking. Some of these include determining whether the search term appears in the META keywords tag, others whether the search term appears in the body text of a web page. A variety of techniques are used to spamdex (see below). Many search engines check for instances of Spiders - Web crawler and will remove suspect pages from their indexes. The rise of Spiders - Web crawler in the mid-1990s made the leading search engines of the time less useful, and the success of Google at both producing better search results and combating keyword spamming, through its reputation-based PageRank link analysis system, helped it become the dominant search site late in the decade, where it remains. While it has not been rendered useless by Spiders - Web crawler, Google has not been immune to more sophisticated methods either. Google bombing is another form of web vandalism, which involves creating pages that directly affect the rank of other sites. Spamdexers may act as consultants, to help other web publishers drive up their sites' ranks using black-hat techniques. Alternatively, they may set up sites of their own that benefit from misleadingly-high rankings -- for instance, creating thousands or millions of landing pages containing links for which the spammer earns a commission whenever the user clicks. Common Spiders - Web crawler techniques can be classified into two broad classes: content spam and link spam.

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