As is the case with trading moving average crosses, buy and sell signals derived from a MACD will come from the crossing of two lines. However, these two lines are not your two EMA lines, rather one is the combined level of the two EMA lines and the second is the signal, or trigger line (the 9 day exponential moving average of the actual MACD itself). The MACD crossing signal line from above would indicate a sell order and conversely the MACD crossing the signal line from below would indicate a buy order.
MACD as a HistogramThe histogram is a visual indication of convergence (moving average lines of MACD moving towards one another) and divergence (moving average lines of MACD moving away from one another).
As the moving average lines cross the histogram will show no lines whatsoever, indicating to traders that lines (prices) may now start in a new direction.