There are four roles that review participants can take up:
Moderator — a person who oversees the review process, resolves disputable situations, and performs double check. The Moderator is also responsible for closing the review.
Author — a person who writes the code that is to be reviewed. The Author is responsible for bugs fixing, in case they were found. The Author acts as the Moderator of the review process in case the Simple Review Workflow is enabled.
Reviewer — a person who studies code and makes comments on it. A Reviewer takes decision whether to accept or reject changes in the source code that were made by Author.
Watcher — a person who monitors a review process. Watchers can receive all review notifications and can join a review any time if they want to add comments to a code.