Some resources here:
A video with pictures: https://www.youtube.com/watch?v=sGfIKmKMRdg
A website with pictures:
http://sphweb.bumc.bu.edu/otlt/MPH-Modules/EP/EP713_CohortStudies/EP713_CohortStudies2.html
Don't worry if this is still confusing. I came across this study in The Lancet Oncology, which describes itself as a retrospective study in the title and abstract, but as a prospective study in the methods section. You are not alone.
Please feel free to add questions or examples below!
All the best,
Jackie