A set of definitions exists in the Senate ESEA bill written and voted out of the HELP Committee (Health, Education, Labor and Pensions).
Reps and sets are terms used by exercise professionals, athletes and individuals in the gym. They provide structure and organization to your workout. If you work with a personal trainer or physical therapist, she will prescribe an exercise based on a specific number of sets and reps.
In most companies, they're just different names for the same role. They're all typically the person who either does outbound calling for an account rep or who answers inbound leads, filtering them for an account rep. The typical progression is the following: BDR, SDR, LDR - development rep. Answers inbound leads, makes outbound calls, generates new opportunities for a rep. Sales Rep, Account Rep, Account Executive - salesperson. does the actual selling, builds longer term relationships, closes deals.