Clone: Clone just copies files to your local computer with no intention of making changes to the repository you cloned
Fork: This allows a user to clone the files, edit them, and then start a pull request in order to try and apply their changes to the source material. Anyone can do this without being a member of the development team so long as the repository is public.
Branch: A branch is very similar to a fork. However, a branch would be created by someone on the actual development team and would add a new repository to the original project instead of downloading it locally. After the work is done in the branch, it would then be merged into the main branch.