Volatility studies are used to predict breakouts. When volatility slows, the expectation is that the market will explode, like a pot of boiling water with a tight lid. Bollinger Bands are an example of a volatility study. Directional studies measure the length and strength of the current trend looking to forecast when the trend will change. DMI, Parabolics and True Range Oscillation are examples if directional studies.
Trend studies indentify support and resistance.
Trend lines and linear regression are two prime examples. Volume studies attempt to measure the buying and selling pressure by weighting price movement in conjunction with volume. On Balance Volume is one example.