Snowdin is the town encountered by the protagonist in the Underground after they leave the Ruins and Snowdin Forest. The town has many guardsmen in it, including Sans and Papyrus, presumably because it is close to the entrance from Mt. Ebott, so it's most likely to have humans coming through. Departure from the town takes the protagonist to Waterfall by foot, or by riding with the River Person, to either Waterfall or Hotland.
Waterfall is the third area in Undertale that the protagonist journeys through, coming immediately after Snowdin and preceding Hotland. It is a dark, wet, and cavernous region, and houses residence for Undyne, Napstablook and their cousin, as well as Gerson's shop. Temmie Village and, subsequently, the Tem Shop is located in Waterfall. By reading the plaques aligned throughout Waterfall, the protagonist learns about the history of Humans and Monsters, as well as information about the nature of their respective SOULs.
Hotland is the fourth region encountered by the protagonist in Undertale, proceeding Waterfall and preceding the CORE. It is the home to Alphys's Lab, as well as many of the game shows of Mettaton. Between Hotland and the Core, the MTT Resort can also be found. The name of the Area is mentioned on the sign during Mettaton's show. It reads: "Mettaton: Live from Hotland."
The MTT Resort is a hotel and entertainment center run by Mettaton at the end of Hotland. It houses a dine-in restaurant and the MTT-Brand Burger Emporium, where Burgerpants works. It is directly connected to the Core of the Underground. Bratty and Catty can be found in the alley to the right of the Resort.
The Surface is the region of the Earth where humans live on. It has been separated from the Underground by the Barrier on Mount Ebott since The War of Humans and Monsters. The only seen portions of the Surface are a city, a beach, and Mount Ebott.