Most countries impose a tax on capital gains. In the UK for the tax year 2011-2012, capital gains tax (CGT) is charged at:
18% or 28% for individuals (depending on your taxable income)
28% for trustees, or personal representatives of someone who has died
10% for gains qualifying for Entrepreneurs Relief
This is a levy charged on capital gains when the profit is realised from the sale of the asset rather than while it is still held.