A programme theory explains how an intervention (a project, a programme, a policy, a strategy) is understood to contribute to a chain of results that produce the intended or actual impacts.
It can include positive impacts (which are beneficial) and negative impacts (which are detrimental). It can also show the other factors which contribute to producing impacts, such as context and other projects and programmes.
Different types of diagrams can be used to represent a programme theory. These are often referred to as logic models, as they show the overall logic of how the intervention is understood to work.