Zones are arbitrary collections of
Rooms, under the control of world authors.
They're mostly used to group Rooms for purposes such as providing
them with common weather, or sharing certain events among them.
For the most part, you can think of them as neighborhoods within a conventional city.
Here's a list
of the neighborhoods that are available and who authored them.
World authors specify the respawn rate of each Zone individually.
Hack-and-slash Zones may respawn quickly, to keep you supplied with opponents.
Others may be very slow.
Further, not all Zones may respawn in the same way.
Some Zones will only ever repopulate if there are no Players inside.
Others may repopulate partially or wholly.
All of this is entirely up to the Zone's author.
Zones may also be used by world authors to limit the movement of mobile NPCs.
NPCs can be flagged by authors to stay inside their Zone of origin.