Neutral Route

The Neutral Route is one of the three primary routes in Undertale, and the one that the majority of players will see on their first playthrough. It is in this route that the final boss is Flowey.

Method
The Neutral Ending is what a player will see if they do not complete the Genocide Route or the True Pacifist Route. To see it, don't kill every single enemy, and do not complete the True Lab.

Neutral Ending Possibilities
In all endings, Asgore dies, Flowey is defeated, the six human souls are lost, the protagonist escapes the Underground and the monsters remain trapped behind The Barrier. There are many versions of this ending all starting with Sans' calling and leaving a message telling the player of the Underground's fate. However, how things turn out after the player leaves depends on who the player befriended, spared, and killed.

Other factors
These only occur if you have not killed Papyrus. Only one can occur, in order of listing here.

If you complete the game without having eaten any food, and are level 1, Sans will ask if you like challenging yourself. If you haven't eaten food but are not level 1, Sans will ask if you've gotten anything to eat.

If you complete the game without ever SAVEing, Sans will ask if you saved.

If the game is completed without taking off the bandage, Sans will ask if you took off "that gross bandage".

If you complete the game in possession of the Snowman Piece, Sans will inform that the player has made a snowman very happy.

Links
Neutral Ending Flowchart