Small, folded pieces of paper shot from rubber bands. Paper tacos are made by rolling a post-it note (or small piece of paper) really tightly, and then folding it in half. These paper "tacos" are then shot from rubber band by placing the "creased" part of the taco in the rubber band, and then flinging the rubber band.
Names vary greatly by region (see tags).
"Ouch! Johnny just shot me with a paper taco."