Monsters

Monsters are a class of creature in Undertale. They dwell in the Underground, beneath the barrier, while Humans live on the surface above the Barrier. Almost every character encountered in the game besides the player is a monster.

Overview
It is claimed in-game that monsters are mostly magic, while humans are mostly water. This is (it is claimed) why monsters are vastly outmatched by humans in a physical combat, which is amply demonstrated by some of the greatest heroes of monsterkind facing off and losing against a single human child.

However, it is claimed that should a monster somehow absorb the soul of a human, the monster will become enormously powerful. Since it is not possible for a human to absorb the soul of a monster (see "Monster souls" below), a human cannot gain power in this manner.

This disparity in potential power was considered a threat by humans, who engaged the monsterkind in a war. While not much is explained or described regarding this war in the game, it must have been rather one-sided, since monsters are physically no match for humans.

Underground
All monsters are trapped within the Underground, ranging from skeletons like Sans, to Spiders like Muffet. All monsters are ruled by the King, Lord Asgore, as well as previously by their Queen, Toriel.

Monster Souls
Monster souls are different from human souls in that they are magical in nature, and comprise the entirety of the Monster's physical being. As such, when they die, they turn to dust and their soul immediately disappears (Boss Monsters excluded). The only monsters who have their souls visibly break are Asgore (by Flowey in Genocide/Neutral Route) and Toriel (by the player in Genocide/Neutral Route).

Additionally, their soul is significantly weaker than a human's.

(Potential Spoilers) * List not complete *

 * Alphys
 * Nice Cream Guy
 * Burgerpants
 * Gerson
 * Onion-san
 * Bratty and Catty
 * River Person
 * Ice Wolf
 * Heats Flamesman
 * Grillby
 * Snowman

(Spoilers Below)

 * Froggit
 * Whimsun
 * Moldsmal
 * Loox
 * Vegetoid
 * Migosp
 * Moldessa (Hard Mode only)
 * Parsnik (Hard Mode only)
 * Migospel (Hard Mode only)
 * Napstablook
 * Toriel
 * Papyrus
 * Snowdrake
 * Ice Cap
 * Gyftrot
 * Doggo
 * Dogamy
 * Dogaressa
 * Lesser Dog
 * Greater Dog
 * Glyde
 * Aaron
 * Moldbygg
 * Woshua
 * Temmie
 * Mad Dummy
 * Shyren
 * Monster Kid
 * Undyne
 * Mettaton
 * Vulkin
 * Tsunderplane
 * Pyrope
 * So Sorry
 * Muffet
 * Royal Guards
 * Final Froggit
 * Whimsalot
 * Astigmatism
 * Madjick
 * Knight Knight
 * Sans
 * Asgore
 * Asriel
 * Amalgamates
 * Jerry