Northern Prairie Wildlife Research Center serves both Moths of North America and Butterflies of North America. The first step in using these two resources to identify a specimen is to decide whether you are dealing with a moth or a butterfly.
Most adult butterflies are diurnal (active during the day) and brightly colored. Their bodies are generally slender and not especially pubescent (hairy). In contrast, adult moths are generally nocturnal (active at night) or crepuscular (active at dawn or dusk). Although some, such as the Io moth are brightly colored or have colorful "eyespots," most moths are drab, with cryptic wing patterns. Bodies tend to be bulky and are often quite pubescent.
If you can get a good look at them, the shape of the antennae is the best way to distinguish butterflies from moths. Except for one group of tropical butterflies, all butterflies have simple antennae that end in a swelling or "club." The clubs may be very pronounced, as in the Nokomis Fritillary or more subtle, as in the White-dotted Cattleheart. Moth antennae range in shape from simple, as in the Rustic sphinx, to feather-like , as in the male White-streaked saturnia moth, but all of them lack the clubbed tip.
Unfortunately, there are no simple characteristics that universally distinguish moth caterpillars or pupae from those of butterflies. Two resources that may help if you are trying to identify a caterpillar are Caterpillars of Eastern Forests and Caterpillars of Pacific Northwest Forests and Woodlands. These cover a broad range of butterfly and moth caterpillars, and together with larval photos in Butterflies of North America and Moths of North America may allow you to identify your specimen or at least narrow it to family. We do not have any guides to pupal identification.