A gusset is an extra piece of material that helps expand or strengthen a fabric object such as a pair of pants, a tote bag or a pillow. A gusseted pillow has side panels, rather than just the top and bottom typical of the average bed pillow.
Spotting the Side Panels
If looking at two different pillows designed for the same purpose -- one with gussets and one without -- the pillow with gussets appears to have side panels or "walls" of fabric stitched between the front and back of a throw pillow or the top and bottom of a bed pillow. Piping might be used as a decorative element around the gussets to make the design stand out even more. Some furniture cushions that look boxy are also gusseted. More fabric is used to make a gusseted pillow than one without gussets that is otherwise the same -- the gusseted version has an added dimension determined by the size of the gusset fabric. Another way to view a gusset is to consider a paper bag made to hold only a greeting card or flat item compared to a paper lunch bag. The paper lunch bag has a gusseted bottom and sides, allowing it to open more fully and stand.