By Chad Langager and Casey Murphy, senior analyst of ChartAdvisor.com
The moving average convergence divergence (MACD) is one of the most well known and used indicators in technical analysis. It is used to signal both the trend and momentum behind a security.
The MACD indicator is comprised of two exponential moving averages (EMA), covering two different time periods, which help to measure momentum in the security. The MACD is simply the difference between these two moving averages, which in practice are generally a 12period and 26period EMA. The MACD is plotted against a centerline along with a nineperiod EMA, which is referred to as the "signal line".
The idea behind this momentum indicator is to measure shortterm momentum compared to longterm momentum to help determine the future direction of the asset.
Calculation
The most commonly used moving average values are 26day and 12day EMAs for the MACD calculation and a nineday EMA of the MACD for the signal line.
These values can be adjusted to meet the needs of the technician and the security. For more volatile securities shorter term averages are used while less volatile securities should have longer averages.
Another aspect to the MACD indicator that is often found on charts is the MACD histogram. The histogram is plotted on the centerline and represented by bars. Each bar is the difference between the MACD and the signal line or, in most cases, the nineday EMA. The higher the bars are in either direction the more momentum behind the direction the bars point.
The buyandsell signals in regard to the MACD are formed either because of crossovers or divergence.
The first signal that is formed by the MACD is when the MACD line crosses the signal line, which is the EMA of the MACD. When the MACD crosses the signal line in an upward direction it is a bullish sign as the asset is gaining upward momentum. When the MACD is gaining upward momentum it reflects that the shorter term moving average (12day) is increasing at a faster rate than the (26day) and that the trend is starting to strengthen in the upward direction.
When the MACD crosses below the signal line it suggests that upward momentum is weakening with downward pressure increasing. It is signals that the shorter term momentum is falling faster than the longer term average, signaling an increase in shortterm selling.
The second signal, which also deals with crossovers, occurs when the MACD crosses the centerline. If the MACD line crosses the centerline in an upward direction it signals upward momentum. This upward crossover can also be seen on the price chart at the exact movement when the 12day moving average is crossing the 26day in an upward direction.
Downward momentum is signaled when the MACD falls below the centerline. This is the point in which the 12day moving average falls below the 26day moving average  a sign of increased downward momentum.
The third signal that is formed by the MACD line is when it diverges from the price movement in the security. This signals that the momentum in the security is moving in the opposite direction of the true trend and suggests a future weakening in the price trend. If the MACD line is moving in an upward direction while the price is moving downward it is a bullish sign. The opposite is true if the MACD line is moving downward while the price is moving upward.
The MACD indicator is by far one of the most well known and commonly used in technical analysis. It is important that anyone using technical analysis become well versed in this momentum indicator.

Trading
A Primer On The MACD
Learn to trade in the direction of shortterm momentum. 
Trading
Spotting Trend Reversals With MACD
Knowing when trends are about to reverse is tricky business, but the MACD can help. 
Trading
Bearish MACD Crossovers With A Positive Twist
These stocks are exhibiting bearish crossovers in their MACD readings, indicating potential shortterm weakness, but also longerterm buying opportunities. 
Trading
Trading The MACD Divergence
Currency traders can use this method to avoid stoporder triggers before the real reversal. 
Trading
MACD And Stochastic: A DoubleCross Strategy
Two indicators are usually better than one. Find out how this pairing can enhance your trading. 
Trading
Measuring Stock Market Sentiment With Extreme Indicators
Pay attention to how the exhaustion principle helps technical indicators signal trend reversals when abrupt value changes coincide with high trading volume. 
Trading
Watch for Bullish MACD Crossovers in These Stocks
These stocks are trending higher but recently experienced a pullback. Watch for a bullish MACD crossover to indicate upward momentum is continuing. 
Trading
MACD And Stochastic: A DoubleCross Strategy
The stochastic oscillator and the moving average convergence divergence (MACD) are two indicators that work well together.