A small cardboard box usually containing juice or fruit drink. They're generally made of paperboard with a lining of aluminum foil. They are normally packaged with a small straw attached to the box; the straw is used to pierce a small plastic-covered opening on the box, from which the juice is, um, sucked. Though they usually contain juice, there are juice boxes designed to hold milk, wine, and even emergency drinking water.
The creation of juice boxes is usually credited to Swedish industrialist Ruben Rausing, whose company Tetra Pak created what was then called the Tetra Brik packaging in 1963. It took a while for this packaging to make it across the Atlantic -- the first juice boxes didn't appear in America until 1980.
The primary consumers of juice boxes are schoolchildren, who take them to school in their lunches, but there are some adults who use them, too -- they fit very well in a lunchbox or lunchbag, and they're a nice treat in the middle of a long workday.