Expert Advisors (EAs) are automated trading systems or algorithms that are designed to trade financial markets on behalf of traders. EAs are commonly used in the MetaTrader platform, such as MetaTrader 4 (MT4) and MetaTrader 5 (MT5), although they can also be implemented in other trading platforms.

EAs can be created using programming languages like MQL4 (for MT4) or MQL5 (for MT5), which are based on C++. Traders can develop their own EAs or purchase them from third-party providers. EAs can be designed to execute trades based on various criteria, such as technical indicators, price action patterns, and fundamental data.

Some key features of Expert Advisors include:

1. **Automated Trading**: EAs can execute trades automatically based on predefined criteria, removing the need for manual intervention.

2. **Backtesting**: Traders can backtest EAs using historical Expert Advisors data to evaluate their performance under various market conditions.

3. **Optimization**: EAs can be optimized to improve their performance based on historical data, although caution is advised to avoid overfitting.

4. **Risk Management**: EAs can incorporate risk management techniques, such as setting stop-loss and take-profit levels, to manage risk.

5. **24/5 Trading**: EAs can trade continuously throughout the trading week, taking advantage of opportunities in different time zones.

While EAs offer several advantages, such as speed, consistency, and the ability to trade across multiple markets simultaneously, there are also risks involved. EAs can be affected by market conditions, technical issues, and programming errors, so it's important for traders to monitor their EAs and use them alongside sound trading principles and risk management strategies.