A forex signal trading service is just a company or individual who provides buy/sell signals to you in exchange for a monthly fee or totally free in some cases. Signal services have some advantages and some disadvantages against expert advisors.
The main advantage and disadvantage enjoyed by forex signal services is that they are mainly managed by humans. Humans take the decisions relative to the trade and then give you the go on the buy or sell orders. This has the advantage that a person has the faculty to consider factors and ea cannot, such as fundamental analysis and other psychological factors. This may improve performance over a single expert advisor. The disadvantage however is that a person or team can be potentially biased by their own emotions. Hence, a forex signal trading service will always be a more “emotional” way of taking decisions than an expert advisor.
The other main disadvantage of a forex signal service is that the vast majority of them require your manual intervention. Meaning that it is necessary for you to execute the trades as instructed by your signal service. This step is completely bypassed when using an expert advisor as it’s decisions are automatically controlled by the software.
Another disadvantage of a signal service is that they generally require a monthly fee, while most expert advisors are purchased for life. This is justified by the fact that the signal service effectively adapts to new market conditions as they happen while an ea is a more rigid structure which must be adapted by construction to a very wide repertoire of market conditions. Ergo, the signal service is more “plastic” than an ea so it is much more likely for a signal service to remain profitable against an ea in the case of a dramatic market event.
In my personal opinion, a forex signal service is just too complicated for automated forex trading. I tried to use several services (which I may post reviews for in the near future) and ended up completely disappointed by the fact that I needed to stress myself over the fact that I had to execute trades even if they came in at 3 a.m. I enjoy expert advisors a lot more as their focus is a much more “set and forget” strategy.