travel comparison websites

Just wondering how the travel comparison websites work, and how does one go about making one?

by travel comparison websites I mean the websites that search other websites to find the different information about a particular flight and hotel and so on,

How can开发者_StackOverflow a website search another website?

any information would be helpful.

Thanks

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Here is some API's, take a look http://blog.programmableweb.com/2007/10/29/5-travel-apis-from-comparison-to-booking/