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Trend Indicator: Bollinger bands
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The Bollinger bands represent a canal around the moving average. The bollinger bands are the moving average of which you add (upper side of the canal) or remove (lower side of the canal) a certain number of time the statistic standard error (generally 2). The more the stock is volatile, the bigger the standard error is.

With:
P_t the closing price at the date t.
MA_tn the  moving average at the date t.
n the number of days.

BA_tn the Bollinger bands

The standard error (std) is:

and the Bollinger bands are:

Interpretation

The interpretation is the same as the one for the moving average but the sale signals are given when the price line (or the short term moving average) cross le bottom Bollinger line from the top and the purchase signals are give when the price line (or the short term moving average) cross le top Bollinger line from the bottom.