A PRI is a digital PSTN interface.  It has 23 B channels and 1 D channel.  In other words, every B channel can be used for inbound or outbound calls at any given time.  At the highest level, it's like having 23 phone lines.

A POTS line is a single, analog telephone line.  They can be configured differently based on your needs but at the highest level, it's a single telephone line.

Depending on the number of users that you have, having a PRI (or multiple) can be much more cost effective than trying to achieve the same level of PSTN connectivity with individual POTS lines.

Hailey