What it measures
Trading volume is the total number of shares for which a buy order and a sell order were matched and executed. For example, if trades of 100 shares occur ten times in a day, that day's trading volume is 1,000 shares.
How to read it, and cautions
Trading volume is often displayed as a bar chart beneath the price chart and is used, together with price movement, as one reference for gauging how active trading has been. However, volume alone cannot be used to judge the future direction of a stock's price.