If you're a U.S. citizen, you'll be considered an "independent contractor" for income tax purposes and no income or other taxes are withheld from the payments you receive.
At the end of the year, any survey company that has paid you $600 or more will send you an IRS Form 1099 - and you're supposed to include this as "miscellaneous income" on your tax return.
If you are not a U.S. citizen, you will need to ask an accountant or other tax professional for the answer to this question, and how it might apply to you in your location.
Please note that we are not accountants and are not qualified to provide accounting, tax, or other legal advice. Our answer is based on our understanding of the tax laws, and how we handle this situation ourselves. Always check with your accountant or other tax professional.