What exactly is capital gains tax and who is affected by it?

Capital gains is defined as income made from the sale of assets that were purchased at a price lower than that of the sale. Capital gains tax would be the taxes the government charges you on that income. Most capital gains taxes are the result of the sale of stocks and bonds, commodities, and real estate. A very good reference for this can be found on Wikipedia at http://en.wikipedia.org/wiki/Capital_gains_tax.