Most large international development research projects, such as those run by DfID, require a theory of change as part of their reporting requirements. A theory of change is a framework that outlines what change the research programme is expected to achieve, how it will achieve that change and how it will measure it. Large GCRF projects which include monitoring and evaluation should use a theory of change approach if appropriate.
Research to Action have produced a reading list on Theory of Change which links to multiple resources. They have also produced a guide on what DfID want in their theory of change.
The Centre for Theory of Change have a glossary of terms used when creating a theory of change.
This video from Aurora Consulting provides an excellent, simple introduction to the concept of Theory of Change.
Another useful starting point for planning a Theory of Change document is available from DIY Toolkit .