This is a Tussock Moth Caterpillar. While the many hairs sticking out from the caterpillar are irritating to some predators, the bristly, white tufts on its back serve another purpose. They are shaped and spaced like the eggs of a parasitic wasp. Normally, these wasps lay their eggs on a caterpillar, so that when the eggs hatch, they have a nice, fresh snack to eat. The tufts make the wasps think that this caterpillar is already taken, so they leave it alone and look for another host for their eggs.