Leave No Trace is a set of principles for people who participate in outdoor recreation that seeks to minimize your impact on the natural environment.
The movement popularized the phrase "Take only photos, leave only footprints."
Back in the 1970s, the United States Forest Service, the Bureau of Land Management, and the National Park Service started to educate their non-motorized visitors (hikers & campers mostly) how to have a minimal impact on the land.
The movement also caught on with the Sierra Club and the Boy Scouts of America. A formal education program was developed in 1990 by the United States Forest Service in conjunction with the National Outdoor Leadership School.
Wilderness areas managed by the U.S. Forest Service actively promote adherence to Leave No Trace principles. Wilderness advocates wanted to protect these areas but limiting wilderness access would probably have actually lost support for the Wilderness Act.
Instead of promoting woodcraft (where wilderness travelers exploit wilderness resources in order to rebel against modern technology), groups went the opposite direction and promoted "Leave No Trace" (where travelers use the latest technology to minimize impact).
The Leave No Trace program is actually managed by the Leave No Trace Center for Outdoor Ethics, a non-profit organization.
Leave No Trace's 7 principles are:
1. Plan Ahead and Prepare
2. Travel and Camp on Durable Surfaces
3. Dispose of Waste Properly
4. Leave What You Find
5. Minimize Use and Impact of Fire
6. Respect Wildlife
7. Be Considerate of Other Visitors