Explainer: Why do reporters need shield laws?

What is a shield law, exactly? When can a government official require a reporter to disclose sources or information? Who counts as a journalist under a shield law? What types of sources or information are protected? Is there a big difference between a subpoena and a search warrant?  by Jonathon Peters CJR
This is a short and useful overview of the legal basis of “privilege” and in particular how journalists’ work is covered (or not) by privilege.