In the United States bonuses are taxed as ordinary income.
Companies may withhold a higher percentage than a standard paycheck, but this is generally because they treat the bonus as if you will continue to receive that income throughout the entire year, and withhold at the corresponding higher rate. When taxes are filed you will receive a refund for any taxes overpaid.
Where I live, tax rate (and income bracket!) is determined month to month, so bonuses are problematic but the company will spread them around to avoid fluctuation.
Companies may withhold a higher percentage than a standard paycheck, but this is generally because they treat the bonus as if you will continue to receive that income throughout the entire year, and withhold at the corresponding higher rate. When taxes are filed you will receive a refund for any taxes overpaid.