Ruins: Abandoned and rotting since Toriel is dead.

Snowdin: a city similar to the cyber city in Chapter 2

Waterfall: A calm place with a couple towns and a small city
Hotland: Industrial powerhouse, consists of factories and stuff

CORE is exploded, a new power source has been built

New Home is like snowdin but less colorful and more dreary and dark, still a big city though, think of say oneshot/