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You should read about screen scraping :-) That's the essence of web crawling.

thank u bro.. Mr jafingi do u have any good refrence of screen scraping

Well, you'd have to understand the fundamentals of DOM: http://www.w3.org/TR/DOM-Level-2-Core/introduction.html

Also learn about XPath, which is often used as a language to selecting nodes in documents.

Scrapy and BeautifulSoup are popular libraries for scraping in Python.