These very common caterpillars are the ones who make the web white nests in wild cherry and other trees along the highways and byways of North America. They can be a pest at times, when there are enough of them to strip nearly every leaf from a small tree.
The caterpillars live communally in the webbing of their nest, and fan out at feeding time to eat the leaves of the host tree. Then they return to the safety of the nest, where they shed their skin, poop, sleep, and just generally hang out until it's time to pupate.
The adult moth is a pretty insect with shining white wings, but most people never notice them!