The Williams %R is an indicator developed by Larry Williams and is similar to the Stochastic Oscillator in calculation but where the Stochastic compares the close to the lowest low over a specified period, the Williams %R compares the close to the highest high over a specified period.
The Williams %R is sometimes called Williams Overbought/Oversold Index.
When prices are trending, Oscillators like the Williams and Stochastics should be viewed with a careful eye when looking at overbought and oversold signals. Generally, when the oscillator is in overbought territory, a crossover into the middle range for the indicator is a signal that prices may fall near term.