GIT: Whats the Difference between a Pull Request and a Merge?

A "pull request" is a request to an upstream repository to merge some changes into their code ("pull changes from my repository and add them to yours, please"). So it's not really like a merge at all -- it's a request to merge.

Yes that is correct, the pull request on GitHub and Bitbucket is a request to merge two branches.

One of the use cases is to have a QA person who is not the developer review and approve the merge request.

Another use case is development on master branch is not allowed. A developer always develop on another branch. Once the development is completed, a pull request is created to merge into master branch.