The MACD (Moving Average Convergence Divergence)is a simple difference between two exponential moving averages of different periods. The MACD is associated with an exponential moving average of shorter period called the signal line.

The commonly used MACD is the difference between an exponential moving average to 26 days and an exponential moving average to 12 days. The signal line is usually an exponential moving average to 9 days.

Purchase signal

La MACD is increasing and crosses the signal line.

Sale signal

La MACD is decreasing and crosses the signal line.

Notes

More the distance between the MACD and the signal line is big, more current trend is strong. It's even more true if the MACD is far from 0.
The MACD is an indicator of overbought (oversold) when the MACD is strongly increasing (decreasing).