A capital gain is the difference between what you pay for an asset and what you sell it for. When you sell something for less than you paid for it is a capital loss. Capital gains taxes take incidental costs into account, so if your selling price is over what you paid, it's a capital gain.
Capital gains tax rates apply to gains you make when you sell an asset. There are specific exemptions to capital gains taxes, such as a family residence exemption. The most important aspect of calculating CGT rates and paying the correct amount on your tax return is organisation.