What are "agency fees" and "right-to-work"?

In light of the Supreme Court's decision to review the case of Janus v American Federation of State, County, and Municipal Employees a little info about the labor practices in question might be helpful.

U.S. Department of Labor -- glossary:
https://www.dol.gov/general/aboutdol/history/glossary

FAQs on unions:
https://www.workplacefairness.org/labor-unions

Agency fees:
https://en.wikipedia.org/wiki/Agency_shop

Right-to-work laws
https://en.wikipedia.org/wiki/Right-to-work_law