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does this means i give $15,000 to them for 3 months for one keyword and my site is listed 24/7 for 3 months?
There is no limit on the number of keywords your can use. And your site will NOT be listed 24/7. It depends on number of factors like popularity of your keywords, number of competitors etc. If your keyword has 5 competitors, the ads will be rotated.
Google's material on Sponsored links is quite self-explanatory
Note that you will be paying CPM rates (unlike Google AdWords which works on CPC model).