A backpacker is a hiker who carries supplies in a backpack. If you hike the Appalachian Trail carrying everything you need on your back, you can call yourself a backpacker.
A backpacker carries her things in a special bag with straps that can easily be worn on her back — a backpack or rucksack. Some backpackers hike through the woods, while others travel from city to city on buses or trains, lugging a backpack along with them. Backpacker comes from backpack, which appeared in the early 20th century as both a noun and a verb.