I'm very new to the area of Causality and I would like some clarity as to the difference between the aforementioned terms.
An intervention is an action or policy applied to one or more units. For example, a smoke cessation program is an example of an intervention. Typically, interventions are thought of as being assigned to units rather than allowing units to choose their intervention participation. Assignment to participation in an intervention is therefore often exogenous, meaning no other variables in the relevant causal system cause participation.
Mediation occurs when the effect of an intervention (or other exposure) occurs through some other variable or mechanism. For example, the effect of the smoking cessation program on smoking behavior might occur through the program's effect on individuals' self-efficacy, which then causes a decrease in smoking behavior. We would say that self-efficacy mediates the relationship between participation in the program and smoking behavior. Mediation is used to describe the mechanisms of an intervention or exposure. Full mediation occurs when the effect of the exposure is fully explained by its effect on the mediator and the mediator's effect on the outcome; partial mediation occurs when not all of the effect of the exposure is explained by the mediator.
In the causal diagram $A \rightarrow B \rightarrow C$, $A$ is the intervention, $B$ is the mediator, and $C$ is the outcome. This diagram represents a randomly assigned intervention and fully mediation by $B$.