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 way of the the River Person, to either Waterfall or Hotland.

Despite being subterranean, is somehow covered in a perpetual layer of ice and snow (thus the name) and because of how cold it is, it's mainly home to monsters with thick fur, such as bears and bunnies. The town also houses the two skeleton sentries (Sans and Papyrus) and the canine members of the Royal Guard (Doggo, Greater Dog, Dogamy and Dogaressa.)

Snowdin Inn
The Snowdin Inn is a building in which the protagonist is able to recover their health above their maximum HP, as stated by the child of the Innkeeper. The price to stay at the inn is 80G, but since the protagonist merely remains in the inn for two minutes, they are refunded. After the protagonist buys a room, they head upstairs, and lie in bed for a few moments. In the adjacent room, three white monsters, sound asleep, create musical snores.

Snowdin Library
The Snowdin Library, spelled as "LIBRARBY" on the sign, is a building near the middle of Snowdin. Inside, the librarian will inform the protagonist that they know the sign is mispelled. Around a table, two newspaper editors are present. The bookshelves in the back are able to be interacted with, and mostly provide game lore. The green bookshelf and the orange bookshelf discuss the nature of SOULs, the light green bookshelf discusses the transition of monsters from Home to New Home, the yellow bookshelf contains an in progress book by the yellow newspaper editor that describes the protagonist's actions, the blue bookshelf compares humans and monsters, and the red bookshelf contains a report on monster funerals.

Main Story
After confronting Toriel and leaving the Ruins, the protagonnist meets Sans and Papyrus in the Snowdin Forest prior to town, where Sans engages in rhetoric with the protagonist, and Papyrus attempts to confront the protagonist with several puzzles.

Neutral Route
After making it through these puzzles, the protagonist makes it into the town, where they pass through and ultimately confront Papyrus in battle. If defeated by Papyrus, they are placed into an easily escaped literal 'Doghouse'. After Papyrus is defeated (via mercy, or losing to him too many times, after which he will give you the choice to battle him or not), the protagonist makes their way towards Waterfall, where along the way they are attacked repeatedly by Undyne, the Captain of the Royal Guard.

Genocide Route
When the protagonist manages to kill everybody in Snowdin Forest, the town of and its buildings will become completely deserted except for Monster Kid, who believes that Undyne will protect them.

Trivia

 * The town's name is a play on the phrase "snowed in".
 * Several cameras can be found throughout the game. Current known ones in are in:
 * The bush directly outside the ruins.
 * The tree behind Papyrus' "sentry station".
 * The lump of snow to the left of the "snowdecahedron".
 * The tree next to the 'dog marriage' sign.
 * The tree behind the sentry station near the dog sculptor.
 * At both ends of the long bridge.
 * The tree in Town straight south from the River Person.
 * These cameras most likely belong to Alphys, as she mentions watching the protagonist through cameras the moment they leave the Ruins.
 * If the protagonist calls Papyrus on the long bridge, he will state that the bridge is actually just a painting over a rock formation.