There are 2 main differences:
1. Psychological. When you trade demo account you are less susceptible to psychological barriers that limit people from profiting in trading. These include fear, inability to stick to trading plan, inability to cut losses, inability to ride profits, etc. When trading in demo account there is no real risk so you are more confident then you actually are.
2. Technical. Sometimes orders will be filled automatically in Demo trading, whereas in real trading you might get other fills or enter late. This is mainly in scam brokers - in IBFX or any trusted brokers you will rarely find such anomalies in quotes.