Registrar
An ICANN-accredited company that sells domain registrations to end-users on behalf of a registry. Customer-facing — you log into your registrar to manage DNS, transfer, or renew.
Top-tier registrars
Cloudflare (at-cost pricing, no upsells), Porkbun (fair prices, clean UI), Hover (simple/expensive), Namecheap (mid-range). GoDaddy is the largest by volume but charges retail premiums.
Switching
Transfer between registrars takes 5-7 days and costs the equivalent of a 1-year renewal at the gaining registrar — your registration term extends by that year.