Short answer: A cafeteria plan, also called a Section 125 plan, is the tax-advantaged structure; an FSA is a benefit that can be offered within that structure; and a POP is a limited type of Section 125 plan that only allows pre-tax insurance premium deductions.
POPs
Premium Only Plans (POPs) are simple Section 125 plans that allow employees to pay their share of health insurance premiums with pre-tax dollars. These FAQs explain how POPs work, who can participate, and what employers need to do to stay compliant with IRS rules.