Forex signal services analyze several factors when preparing their data. They do a technical analysis of market conditions and use a combination of indicators to identify trends and isolate profitable entry and exit points. They then send you the results via the venue of your choice and you can choose to use the signal in your own trading, or pass on it.
Most forex signal services offer signals for only a handful of the most popular currency pairs, such as EUR/USD, USD/JPY, GBP/USD, USD/CHF. Occasionally, you can find specialty services that offer signals for other lesser traded pairs. Forex signals can be costly, even upwards of $100 / mth. The benefit of subscribing to such a service is that they analyze and crunch the data for you, saving you time. It should be noted, however that using a signal service is no substitute for a proper education in the Forex markets. Signal services give you data, you still need to know what to do with it.
When shopping for a signal service, make sure that they provide you with historical data so that you can see their track record for yourself. Remember, that like any trader, Forex signal services also have loosing trades.