Trading Volume and Open Interest

How should trading volume http://www.financial-spread-betting.com/course/volume-technical-analysis.html influence your trading? You can see price patterns but you need evidence to tell you how significant price patterns are. Volume tells you how significant a pattern is. Rising volume endorses a move - falling volume throws a question mark on a move. Rising open interest enforces a move, falling open interest throws question markets. What is open interest?