What is backpacking?

A backpacker packs all of his or her gear into a backpacK. This gear must include food, water, and shelter, or the means to obtain them, but very little else, and often in a more compact and simpler form than one would use for stationary camping. A backpacking trip must include at least one overnight stay in the wilderness (otherwise it is a day hike). Many backpacking trips last just a weekend (one or two nights), but long-distance expeditions may last weeks or months, sometimes aided by planned food and supply drops.
Backpacking camps are more spartan than ordinary camps. In areas that experience a regular traffic of backpackers, a hike-in camp might have a fire ring and a small wooden bulletin board with a map and some warning or information signs. Many hike-in camps are no more than level patches of ground without scrub or underbrush. In very remote areas, established camps do not exist at all, and travelers must choose appropriate camps themselves.
In some places, backpackers have access to lodging that are more substantial than a tent. In the more remote parts of Great Britain, bothies exist to provide simple (free) accommodation for backpackers. Another example is the High Sierra Camps in Yosemite National Park. Mountain huts provide similar accommodation in other countries, so being a member of a mountain hut organization is advantageous (perhaps required) to make use of their facilities. On other trails (e.g. the Appalachian Trail) there are somewhat more established shelters of a sort that offer a place for weary hikers to spend the night without needing to set up a tent.
Most backpackers purposely try to avoid impacting on the land through which they travel. This includes following established trails as much as possible, not removing anything, and not leaving residue in the backcountry. The Leave No Trace movement offers a set of guidelines for low-impact backpacking ("Leave nothing but footprints. Take nothing but photos. Kill nothing but time"...)


PROFESSIONAL BACKPACKING

For some people, backpacking is a necessary and integral part of their job.
In the military a framed backpack is referred to as a "rucksack" or simply a "ruck". Soldiers who serve in the militaries of most nation-states usually receive at least some rudimentary backpacking training while infantrymen are often trained to a more advanced backpacking skill level. They share many common attributes with amateur backpackers: being self-contained, use of land-navigation skills and actively minimizing their environmental foot-print. Although there are also a few differences such as the need to carry an assault rifle, other weapons, ammunition and communication equipment as well as at times maintaining "noise and light discipline", which means remaining silent and in darkness to avoid detection.
Other professional backpackers may be scientific and academic researchers, professional guides, photographers, park-rangers and "search & rescue" personnel.

WINTER BACKPACKING

Although backpacking in the winter is rewarding, it can be dangerous and generally requires more gear. Backpackers may need skis or snowshoes to traverse deep snow, or crampons to cross ice. Cotton clothing, which absorbs moisture and chills the body, is particularly dangerous in cold weather, so backpackers stick to synthetic materials or materials that won't hold moisture. Special low-temperature sleeping bags and tents can be expensive, but will be more comfortable than many layers of warm clothing. However when hiking in cold weather it is always better to hike with varying layers of clothing so that as the body heats up layers can be taken off without causing the wearer to sweat or become very chilled.