In the United States, individuals and corporations pay income tax on the net total of all their capital gains just as they do on other sorts of income, but the tax rate is lower for "long-term capital gains", which are gains on assets that had been held for over one year before being sold. The tax rate on long-term gains was reduced in 2003 to 15%, or to 5% for individuals in the lowest two income tax brackets. Short-term capital gains are taxed at a higher rate: the ordinary income tax rate. In 2013 these reduced tax rates will "sunset", or revert back to the rates in effect before 2003, which were generally 20%.
Technically, a "cost basis" is used, rather than the simple purchase price, to determine the taxable amount of the gain. The cost basis is the original purchase price, adjusted for various things including additional improvements or investments, taxes paid on dividends, certain fees, and depreciation.
Exemptions from capital gains taxes in the United States include: