What is the difference between butterflies and moths?

Butterflies are mostly brightly colored day-flying insects with long clubbed antennae. 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."