The advance/decline line is the most popular of all internal indicators by far. It is a very simple measure of how many stocks are taking part in a rally or sell-off. This is the very meaning of market breadth, which answers the question, "how broad is the rally?" The formula for the advance/decline line looks like this:
A/D Line = (# of Advancing Stocks - # of Declining Stocks) + Yesterday's A/D Line Value
The most popular data used for the A/D line is from the NYSE or Nasdaq markets. It is cumulative and normally plots a line similar to the price chart of the given index. The A/D line can be used alone or together with the price chart to look for divergences. A divergence suggests that a move in the price chart is unsupported by the broad market, and it should, therefore, be taken as a warning of an impending turning point in the index or market.
More information on the A/D line is available HERE
(Information provided by Investopedia)