Moving Average Convergence Divergence or MACD measures the difference between two Exponential Moving Averages or EMAs.
EMA's are a type of moving average where the most recent data is given more importance and thereby, are able to more accurate reflect recent movement in the market.
For example:
Positive = 12 day EMA > 26 day EMA
Negative = 12 day EMA < 26 day EMA
Keep in mind, moving averages are lagging indicators; they track what has already taken place and because MACD uses absolute differences rather than percentage changes. It doesn't provide a sensitive measure over long periods of time.
Book: The Advanced Moving Average Convergence-Divergence