Demo accounts help traders test their strategies without using real money; the trading conditions are identical to Real accounts but the currency units used are not backed by real money.
All account types have Demo accounts available, except the Standard Cent account which can only be traded on with a Real account.
Please note that there is a limit to how many Demo accounts you can have per account type and that you may want to learn more about the differences between a Real account and a Demo account to find out which is best suited for your requirements.
Follow the link for a simple guide on how to register a new Demo account.