Here's another set of concepts that seem to cause a great deal of confusion. They are much used in results-oriented planning (often called results-based management or RBM). I like to explain them as follows:
Output = The direct result of an activity - something that is under your/ your project's control. For instance, I brush and floss my teeth several times a day, and the output is a clean set of teeth.
Outcome = Something that your activity is designed to help produce - but it takes some more factors for that kind of result to come about. For instance, I clean my teeth to avoid getting caries, so healthy teeth are my desired outcome. But my chances to have good teeth are much enhanced if I avoid eating sweets or very acid food, if I have healthy gums, if I have the right kind of genes, and so on. Even people with clean teeth get caries.
Impact = A long-lasting result that can be directly traced to an intervention. For example, if my dentist extracts a tooth, the impact is a gap in my mouth.