This is a phone network over which telephone service providers route all phone calls over a specifically built network using 'switching' technology.

This refers to the worldwide 'plain old' telephone system used for exchanging phone calls.

This is a technology that carries calls over the Internet or through any other Internet Protocol-based network, instead of going through the PSTN.

Note that the main distinction between these two types of telephony services, from a consumer's perspective, is that VoIP services require mains electrical power supply and an internet connection in order to work, whereas the PSTN does not. This means that, with a VoIP service, in the event of failure in the main electrical power supply, you will require a power battery backup or an uninterruptible power supply (UPS) to ensure continuity of service. Note, this also applies to customers using cordless phones, as these phones also require an electrical power supply to recharge and operate.