Lepidoptera are classified into butterflies and moths. Taxonomists commonly argue over how to define the obvious differences between butterflies and moths.
Shape of antennae
The most obvious difference between moths and butterflies is in their feelers, or antennae. Most butterflies have thin filament-like antennae that are club-shaped at the end. One the other hand, moths often have comb-like or feathery antennae. This distinction is the basis for the earliest taxonomic divisions in the Lepidoptera - the Rhopalocera ("clubbed horn", the butterflies) and the Heterocera ("varied horn", the moths).
There are, however, exceptions to this rule and a few moths (for example, Castniidae) have clubbed antennae. Some butterflies, like Pseudopontia paradoxa from the forests of central Africa, lack the clubbed ends.
Wing coupling mechanisms
Many moths have a frenulum which is a filament arising from the hindwing and coupling with barbs on the forewing. The frenulum can be observed only when a specimen is in hand. Some moths have a lobe on the forewing called a jugum that helps in coupling with the hindwing. Butterflies however lack these structures.
Moth vs Butterfly Pupae
Most moth caterpillars spin a cocoon made of silk within which they metamorphose into the pupal stage. Most butterflies on the other hand form an exposed pupa which is also termed as a chrysalis.
However there are many exceptions to this rule, for example the Hawk moths form an exposed chrysalis which however is underground. Gypsy moths sometimes form butterfly-style pupae, hanging on twigs or tree bark, although usually they create flimsy cocoons out of silk strands and a few leaves, partially exposing the chrysalis. A few Skipper butterfly larvae also make crude cocoons in which they pupate, exposing the pupa a bit. The Parnassius butterfly larvae make a flimsy cocoon for pupation and they pupate near the ground surface between debris. Sometimes, parasitic wasps lay their eggs inside the body of the caterpillar. Once the larvae come out of the caterpillar, the caterpillar spins its cocoon around the larvae instead of itself, and dies protecting the larvae of other species.
Coloration of the wings
Most butterflies have bright colors on their wings. Nocturnal moths on the other hand are usually plain brown, gray, white or black and often with obscuring patterns of zigzags or swirls which help camouflage them as they rest during the day. However many day-flying moths are brightly-colored, particularly if they are toxic. A few butterflies are also plain-colored, like the Cabbage White butterfly.
Differences in body structure
Moths need to conserve heat during the cooler nights so they tend to have stout and hairy bodies. Moths also have larger scales on their wings which makes them look more dense and fluffy.
On the other hand, butterflies are able to absorb solar radiation. So they have slender and smoother abdomens. Butterfly scales are finer than moth scales.
Time of activity
Most moths are nocturnal or crepuscular while most butterflies are diurnal. Exceptions to this rule include the diurnal Gypsy moth and the spectacular "Uraniidae" or Sunset moths.
Moths usually rest with their wings spread out to their sides. Butterflies frequently fold their wings above their backs when they are perched although they will occasionally "bask" with their wings spread for short periods. However some butterflies, like the skippers, may hold their wings either flat, or folded, or even in-between (the so-called "jet plane" position) when perched. Most moths also occasionally fold their wings above their backs when they are in a certain spot (like when there is no room to fully spread their wings). A sometimes confusing family can be the "Geometridae" (such as the Winter moth) because the adults often rest with their wings folded vertically. These moths have thin bodies and large wings like many butterflies but may be distinguished easily by structural differences in their antennae (e.g. bipectinate).