Web crawler is more commonly known as a Web spider or a web Bot.The crawler is automated program that is designed to scan the World Wide Web. These crawlers are used by search engines like Google, Yahoo or Msn. The sites are keep changing that may be its content etc. so all this information, crawler gives to the search engines.
The crawler begins with the list of websites to be scan is known as seeds. When crawler comes to the site it first identifies the hyperlinks on the page and then puts them on the another list of the URL’s called crawl frontier. There are a couple of factors that may affect the way a crawler crawls a site. If a website is large then it takes time for crawler to scan the website but if the website is small then it takes less time to scan.
Another thing is that crawler will see if the site has undergone any changes since it visited last time. There may have some pages which are added or removed. Crawlers want to see every time some new in the website. It can take a crawler weeks or a few months to change a rank but in that time there could be changes that have been made to the site. The crawler will already have taken this into consideration. You may have added or removed some content. Try to keep the site the same until your index changes. You can add pages because search engines love new content.
Crawler will visit only those sites which are end in .html, .htm, .asp, .php and a slash. Stay away from MIME type.
Crawler are programmed in such a way that they can figure it out the content of the site is fresh or not. They are such a smartly programmed that you can’t make them fool. They can determine the freshness and the age of the information. Some of the different types of crawlers are the RBSE, the Web Crawler, The World Wide Web Worm, the Google Crawler, Cobweb PolyBot, Mercator and the Fast Crawler. You can expect to get a visit from the crawler at least every three months. Some people believe that if you already have a good ranking that it won't change your rank but it can and has over the years. Nothing is guaranteed on the web.
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