Since the question came up in a comment earlier this week, I thought I would post the answer I found when I looked it up.
Excerpt: “The physical differences between moths and butterflies are usually easy to see: Butterflies’ antennae are wider at the tips — their ends look like little clubs….Moths’ antennae are often feathery. Sometimes, they’re thin like butterflies’ antennae, but without the clubs. Most of the time, butterflies’ wings display more vivid colors than moths’ wings do….Many moths use a series of loops called a retinaculum and a fringe called a frenulum to connect their front and rear wings. Butterflies don’t have these structures…Often, moths’ bodies are plumper and fuzzier than butterflies’ bodies.”