Leave No Trace is an ecological principle of leaving an environment of habitation in such a condition as to render it impossible for future observers to discern the previous presence of the practitioners of the Leave No Trace methodology.
The slogan "Leave no trace" (LNT) summarizes a trend among hikers to address concerns about conservation of the ecosystems and scenery that often motivate hikes. An organization (http://lnt.org/) of the same name promotes LNT practices in cooperation with the NOLS and the AMC.
One simple and fundamental aspect of LNT is "If you can carry it in, you can carry it back out"; a particularly blatant target of this principle is the popular disposable plastic bottle of water, which weighs a pound or so, and has a volume that challenges many pockets. Once empty, with its top left loose, it is a five-second project to convert it into a negligible weight occupying a back pocket, instead of a slow-to-decay eyesore.
Another way to put it is: "Take only pictures, leave only footprints" (a motto of ecotourism).
There's actually a simple list of tenets that anyone can follow to be sure to Leave No Trace when enjoying the outdoors.
Less is More
Camping is for most of us a very social activity, but the sense of solitude and "oneness" with nature is an equally important aspect of the experience. To that end, try to keep your group as small as reasonably possible. Not only will this enhance your interaction with nature, but it will minimize your group's impact on the wilderness.
Similarly, be somewhat conservative when making a packing list, especially when it comes to food. Not only will your back and legs thank you later on for not having to haul around the extra weight, but it will minimize the amount of trash you risk leaving behind. For instance, stripping the excess packaging off of the food you bring along can save ounces or pounds of weight, and save you the trouble of having to pack it out again, or (worse yet) accidentally leaving it behind!
Also, although it may seem counter-intuitive at first, bringing an extra pair of casual shoes or sandals along can be quite useful. Not only are sandals much less damaging to the vegetation and ground cover around camp, but they can come in handy in a pinch, allowing you to, for instance, cross a rocky stream without having to soak the hiking boots you have to keep on 8 hours a day for the next week! Not to mention, it's nice to be able to relax your feet after a long day of hiking in an airy pair of sandals or shoes.
When it comes to cooking (and toiletries, for that matter), make sure to bring a concentrated, bio-degradable, and multi-function camp soap along. One 2-ounce bottle of camp soap can eliminate the need for pounds of shampoo, dish soap, hand/body/face soap, and laundry detergent. Also, whenever possible, cook on a stove rather than an open fire. Campfires do tremendous irreperable damage to their surroundings, leaving scorched and stained rocks, burnt land and vegetation surrounding the fire pit, consuming natural materials, and polluting the air with smoke. Even in built-up campgrounds, open fires are often banned these days for just these reasons, and rangers at all park levels will often investigate any unidentified plume of smoke.
Finally, carry the smallest tent necessary to provide you protection against the elements. A larger tent, although more comfortable, means more weight for you to carry around all day, and a larger footprint left behind when you break camp.
Consider your Surroundings
This especially applies when choosing a campsite; hardened or barren ground should always be chosen over grassy and overgrown areas in order to prevent any further damage to your surroundings. Waterfront sites should be avoided at all cost! Not only do they harbor the most fragile ecosystems, but oftentimes the water can rise unexpectedly during the night due to wind, waves, tides, or rainfall upstream. This can actually be extraordinally dangerous in some areas, including hill country, deserts, and canyons, where water levels can rise dozens of feet in just a few minutes, catching unsuspecting campers off guard.
Also be considerate when disposing of waste of any kind. Any degradable human waste (including greywater) should be disposed of at least 200 feet from the nearest trail, campsite, stream, or other fragile natural resource, and buried whenever possible. Although unpleasant, items such as used bandages, toilet paper, and tampons should always be packed out, never buried. Many areas are so sensitive that feces as well is required to be packed out.