Both moths and butterflies are in the order Lepidoptera, but there are general differences that can help you know which is which.
Here are a few overall rules that can be used to distinguish moths from butterflies. Of course, there are always exceptions to the rules.
Moths have simple thread-like or ‘feathery’ antenna without a club.
Butterflies have a thickened club or hook on the tip of the antenna.
Exceptions: Several families of moths have antennae with clubs, most notably the Sun moths (Castniidae).
Moths typically have duller colors.
Butterflies usually have brighter colors.
Exceptions: Many moths are brilliantly colored, especially day-flying moths.
Moths hold wings flat when resting.
Butterflies hold wings together above their body when resting.
Exceptions: Many moths, including Geometrid moths (Geometridae spp.) hold their wings up in a butterfly-like fashion when resting. Butterflies in the Lycaenid subfamily Riodininae, and Skippers in the subfamily Pyrginae hold their wings flat when resting.
Moths spin a cocoon before they pupate.
Butterflies will shed their skin for the last time and reveal a chrysalis.
Exceptions: Many moths do not spin a cocoon; many butterflies and skippers form a silken shelter, often with plant leaves.
Moths are nocturnal and fly at night.
Butterflies are diurnal and are active during the warmth of the day.
Exceptions: A few butterflies are active at dusk; many moth species fly during the day.