Differences and similarities between mediation and moderation
The most obvious similarity is that both methods involve three variables (in the simplest case). There is a clear IV to DV relationship in both methods that is examined too. A third similarity is that multiple regression can be used in both methods.
The most obvious difference between the two methods is the intended conclusion. For moderation, one wants to identify whether a significant interaction has occurred, whereas in mediation the issue is whether there is overlapping variance among the three variables that permits a causal statement. The statistical techniques involved in these two methods are different to some extent too. Although regression can be used in both techniques, the types of regressions are different insofar as the type and order of independent variables varies between the two methods.
In short, it is easy to understand why users are confused between these two methods. There is significant overlap between the two methods, but at the heart of the matter, they are quite different in their goals, their enactment, and in the resulting interpretations.