True Pacifist ending: Don't kill anyone, befriend Papyrus, Undyne, and Alphys (only possible after getting a neutral ending, you'll need to get a letter from Undyne to initiate this), and clear the True Lab. Note: You can do this without starting a new game as long as you haven't killed anyone, just load after getting the neutral ending, then fulfill the conditions for this ending.
Genocide ending: In each area, kill every single encounter until no one pops up on random encounters, and kill all bosses. WARNING: Will taint future true pacifist endings... Unless you tinker with the game's save files (and probably registry entries).
