Beavers are amazing animals for many reasons, but their building ability is top-notch. They are actually one of the few animals other than humans that are able to modify their habitat. They do this with all their building activity.
Busy beavers build both dams and lodges. The lodges are their homes, and they typically have multiple underwater entrances and a living space that is above water. They often build their lodges in ponds created by their dams. They build dams across streams or other waterways to create a pond for themselves to live in, but the ponds also create homes for lots of other wildlife too.
Beavers are considered a keystone species because their presence is so important to the ecosystem. If they left a habitat area, many other plant and animal species would be affected.