Think of a POS system as the convergence point where a customer and a business make a deal and concluded a deal.
The words "check out" or "register" may be used to refer to the POS, but these are simply stations where the transactions happen.
Modern POS vary widely, from hardware for scanning credit cards that connects directly to your smartphone, to complete systems with terminals and cash drawers.
Whether simple or complex, today's POS allow merchants to ring up a purchase in seconds, and do it all electronically.
Sometimes the term POS may be used interchangeably with the term point of service, as customers may use the same technology to make returns.