A backpack is a bag you wear on your back, with straps over your shoulders. Lots of kids carry their books to school in a backpack.
A hiker might carry a heavy backpack for miles, with food and a folded tent inside, while a kindergartner's backpack will be much smaller, and may have a cartoon character on it. You can also call this portable kind of bag a rucksack. If you hike with a backpack, you go backpacking, and you can call yourself a backpacker. The word's been around since the earliest 20th century.