The term julienne means to cut vegetables into thin, even, matchstick pieces. You can use a food processor to julienne vegetables such as carrots and zucchini -- attach the blade provided for julienning, cut the vegetables into pieces that will fit into the feeder, and turn it on -- but the results are not as precise as doing it with a mandoline or by hand.
A mandoline is a kitchen tool with adjustable blades for thick or thin slicing. On most machines, the food is held in a metal or plastic carriage as it is run over the blades to protect your fingers. The French mandoline is a heavy-duty stainless-steel machine that provides the most options for cuts, including waffle cuts and crinkle cuts as well as julienning. The Japanese mandoline is made of plastic and is less expensive; it performs fewer types of cuts, but does julienne quite well. It does take longer to julienne with either type of mandoline than to use a food processor, but the pieces cut by a mandoline are longer, more uniform, and more beautiful.