The main difference is that you will trade will real funds with real accounts. While in demo accounts, you will use virtual funds with no real value to trade.
Although all features and functionality are the same on both accounts, you should remember that simulation cannot replicate actual trading market conditions. One significant difference is that the volume executed via simulation does not affect the market. In contrast, the real trading volumes affect the market, mainly when the deal size is vast. However, the execution speed is the same for both the demo and real accounts.
Furthermore, traders can have a diverse psychological profile when trading demo or real accounts. This may affect the evaluation of your performance when trading demo. Therefore, it is vital to be cautious; not to be complacent and draw conclusions from using demo accounts.
