The majority of services use a combination of factors in identifying trends in the market and in recommending entry and exit points, but all are based in the main on a technical analysis of the currency markets. These services in essence compile currency charts and then use a variety of mathematical models to make their trading recommendations.
For example, they may use a simple moving average to trigger buy signals as currency prices move above the average line and sell signals as prices fall below the moving average. In addition, volume indicators can also be used to indicate levels of interest in the market with high volume, especially when it occurs close to the bottom of the market, indicating the possible start of a new trend and low volume pointing to investor uncertainty.
This of course is a somewhat simplistic picture used here only for illustration of the nature of Forex signals. In reality a large number of tools are used, including those already mentioned and many others such as Bollinger bands and volatility and momentum, and these together form part of a complex mathematical model which generates the signals sent out to subscribers.
Services will of course vary considerably, as with anything else in life, and they are very much an aid to the busy trader and just one tool in his toolbox. They are certainly not infallible and only your own experience of using such services will really determine whether or not they are of sufficient benefit to you to warrant the cost of anywhere from about $50 to $200 a month.
One important point to remember is that Forex signal services provide you with advice and nothing more. It is up to you to take that advice and act upon it or not as your own knowledge and experience tells you. If you simply take the advice provided by the service and act upon it blindly then, if you have a very good service, you may come out on top but, in many cases, you will find that your trading is less than successful.