A woodchuck is a small, mostly nocturnal mammal in the genus Marmota of the marmot family. The woodchuck is native to North America and is found in temperate regions throughout most of Canada and the United States, but not Alaska or Hawaii.
The main difference is that woodchucks are mammals, whereas groundhogs are not. Groundhogs are in the same family as rabbits (which are also rodents), but they're not mammals. A groundhog is more like a squirrel than it is like a rabbit. Squirrels can climb trees, they are nocturnal, and they have opposable thumbs.
A groundhog can be seen in daylight.