Williams %R
(Revised July 01, 2011)
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In these tutorials I will discuss what to do when you visually look at a technical indicator or oscillator on a chart, I won't discuss math jargon or history, I think it's more important to know how to react at a glance (what is the chart saying), therefore only consider this an introduction in brief based on my opinion.  You should educate yourself further if interested. The charts used are not invitation to buy or sell, and are used only for example purpose.
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Intro
With the William %R  indicator we get very easy to see "buy or sell" signal based on a stock being oversold or overbought.
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So how does it work?
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Williams Buy Signal is when the Williams indicator is below the oversold (20) line and it rises to cross over the (20)line.
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Williams Sell Signal is when the Williams indicator is above the overbought (80) line and then falls below the (80) line.

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Above is what to look for when using Williams %R with a non trending stock (trading sideways):
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Below is how to use William %R to detect a stock in a strong trend:

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As the above chart illustrates, when the Williams % R indicator stays in the oversold area (below 20) and any bullish rally barely registers with the Williams %R(i.e. fails to go above 80), then the downtrend is strong and a trader should not go long on the stock.
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Similarly, when the Williams %R indicator stays in the overbought area (above 80) and any attempt at a downturn fails to send the indicator into oversold territory (i.e. fails to go below 20), then the uptrend is strong and a trader should not go short. 
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So what do I think?
Although the Williams %R is very easy to interpret, it is more useful during sideways trading (non-trending markets).  During trends the Williams %R can give a false signal which can lead to losses.  Nevertheless, the Williams %R does give significant signs of strong trends that can easily be identified by a trader for profit.
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Unfortunately many chart systems may label and even draw Williams %R differently as you can see below (don't ask why), just note the above principles always remains the same.
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source: google.com (0 is the sell/overbought, 100 is the buy/oversold)
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source: bigcharts.com (0 is the sell/overbought signal, -100 is the buy/oversold) 
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Remember, not one technical indicator should be used by itself.  The more indicators that line up to confirm the signal the better chances your trade will succeed.
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Good luck on all your trades and or investments
And happy capitalism