I have earned some money from a US based website and now I want to withdraw my funds but they are saying to do that I need to fill w8BEN form? Is it that I will be charged any tax by US government? Also since there is a treaty between our country(India) and United States, is it possible to get a tax rebate?

Are you in India? The U.S. company will regard this as income earned in the U.S. and will deduct withholding taxes.
There is no tax treaty benefit available.

However, if the income earned is LESS than $3,500, you will get the withheld money back as a refund when you file yornon-residnet atx return (Form 1040NR or 1040NR-EZ).