The name says is very heavily related to the subject
Ruins
This song plays in Snowdin Town
Snowdin Town
This song plays while you run away from Undyne
Run!
This song has the longest title in the Undertale Soundtrack
Can You Really Call This A Hotel I Didn't Receive A Mint On My Pillow Or Anything
This is the most popular song on the Undertale Soundtrack
MEGALOVANIA
This song plays in Toriel's house and has a music box version
Home
This is Papyrus's main theme
Nyeh Heh Heh!
This song plays in a secret village
Temmie Village
This is Mettaton EX's fight theme
Death By Glamour
The first song in the game
Once Upon A Time
This song often causes anticipation to new players
Anticipation
This is the first song you hear in Snowdin
sans.
Mad Dummy's theme
Dummy!
This is Mettaton NEO's fight theme
Power of NEO
This song is ignored completely by the Untertale community
Bring It In, Guys!
This is Toriel's battle theme
Heartache
This song plays in the room where you face Glyde
Mysterious Place
This song plays during the Undyne the Undying fight
Battle Against A True Hero
This is Mettaton's fight theme
Metal Crusher
This was originally going to be in the Sans fight, but never actually appears in the game
Song That Might Play When You Fight Sans
This song plays when Toriel tests your independence
Unnecessary Tension
This song plays when Sans tells you about Flowey talking to Papyrus
Premonition
This song plays when you first meet Undyne
Undyne
This is the theme of Hotland's secret boss
Wrong Enemy!?
This song never made it on the Undertale Soundtrack and doesn't appear in the game normally, but the character is said to have AT 666 and DF 666
Gaster's Theme