Volume is my main indicator, it accounts for about 90% of my Technical Analysis, just to give you an idea about how important it is.
This is the definition, taken from here:
http://stockcharts.com/school/doku.php?id=chart_school:glossary_vVolume - The number of trades in a security over a period of time. On a chart, volume is usually represented as a histogram (vertical bars) below the price chart. The NYSE and Nasdaq measure volume differently. For every buyer, there is a seller: 100 shares bought = 100 shares sold. The NYSE would count this as one trade and as 100 shares of volume. However, the Nasdaq would count each side of the trade and as 200 shares volume.