What are Public Lands?
Public lands are lands managed by the federal government for the use and benefit of people and for natural resource conservation. Public lands primarily include lands administered by the "Big Four" land management agencies - the National Park Service and it's National Park System (dark green on map below), the U.S. Forest Service and its National Forest System (light green); the U.S. Fish & Wildlife Service and its National Wildlife Refuge System (orange); and the Bureau of Land Management and its lands (yellow).
Among these "Big Four" agencies, the U.S. Forest Service and Bureau of Land Management have a much larger focus on resource extraction (timber for the national forests; grazing and mineral extraction for the BLM) than the National Park Service and U.S. Fish & Wildlife Service, but it is not black and white. All four land systems have certain areas Congressionally designated as wilderness, which is the highest form of preservation a landscape can have.
Beyond the agencies and land systems mentioned above, in my mind, there are two other entities that should be included in the umbrella of public lands. First of these are state parks, regardless of the state they are in. State parks are generally set aside for outdoor recreation and natural or cultural history preservation. They are managed by individual states rather than the federal government. American Indian lands, or reservations, particularly the parks on them which indigenous people allow and encourage outsiders to go to and enjoy, can also be considered public lands in my mind.
Above Photo: Blue Hole, Key Deer National Wildlife Refuge, Florida