The main difference is that with real accounts you will trade with real funds, while demo accounts use virtual money with no real value to trade.
Other than that, the market conditions for demo accounts are exactly the same as they are for real accounts, making them ideal for practicing your strategies.
While all features and functions of a real account are also available for a demo account, you should keep in mind that simulation cannot replicate real trading market conditions. The speed of execution is the same for real trading accounts as for the demo accounts.