For example we have 2 really popular free systems (3 Pigs and PAST).  Many people are successful using them adapted to their own style of trading.  It is highly likely not one of them would ever have the exact same entries as we (the authors) do but they can and will be as successful by using and applying the principles in a manner suitable to them.  E.g. one of the strategies is a 4 Hour MTF strategy where the original system states a Trailing Stop based on an MA with an open Target.  One trader is now using this very successfully (and reports the trades on Babypips) us...
Arbitrage is based on the premise of the forex trader trying to make a gain from small differences (of the currency) that exist either in the same or different markets. This is primarily a form of speculation. Identifying the right conditions and employing this strategy is not an easy task. Arbitrage strategy best market participants who have best technology systems and have quickest access to information. Arbitrage is best employed when the same currency has two different prices.

Apart from the entry/exit rules and optional money management guidelines, strategies are often characterized by the list of trading tools required to employ the given strategy. These tools are usually charts, technical or fundamental indicators, some market data or anything else that can be used in trading. When choosing a strategy, you need to understand, which of the required tools you have in possession.