Butterflies are mostly brightly colored day-flying insects with long clubbed antennae and most moths fly at night and lack clubs at the end of their antennae. A group of tropical "moths" has been found that are closely related to butterflies but they lack clubs on their antennae; they are now considered to be butterflies. Perhaps the best answer that matches our current knowledge is just to say that butterflies are "fancy moths."

An Eastern Tailed-Blue (Cupido comyntas) butterfly with clubbed antennae.	A Pacific Orangetip (Anthocharis sara) butterfly with clubbed antennae.
Cycnia tenera moth, lacking clubbed antenaae.	Haemtopis grataria moth, with comb-like antennae.