A backlight unit comprising: a first light source (210; 340, 341; 511; 811) configured to provide first light for a three-dimensional, 3D, display; a second light source (220; 513; 640; 813) configured to provide second light for a two-dimensional, 2D, display; a light guide plate (130; 310; 520; 610; 820) comprising: a first output pattern (131; 230; 311; 411; 611; 711) formed on an upper surface of the light guide plate (130; 310; 520; 610; 820) and configured to output the first light to a display panel (120; 330; 630); and a second output pattern (132; 240; 312; 412; 612; 712) formed on a lower surface of the light guide plate (130; 310; 520; 610; 820) and configured to output the second light to the lower surface of the light guide plate (130; 310; 520; 610; 820), wherein the first light source and the second light source are disposed on different side surfaces of the light guide plate that do not face each other; a reflector (140; 325; 625; 720) configured to reflect light output to the lower surface of the light guide plate through the second output pattern to the display panel; and a controller configured to control the light sources to output light for a 2D display mode or a 3D display mode. a first light source (210; 340, 341; 511; 811) configured to provide first light for a three-dimensional, 3D, display; a second light source (220; 513; 640; 813) configured to provide second light for a two-dimensional, 2D, display; a light guide plate (130; 310; 520; 610; 820) comprising: a first output pattern (131; 230; 311; 411; 611; 711) formed on an upper surface of the light guide plate (130; 310; 520; 610; 820) and configured to output the first light to a display panel (120; 330; 630); and a second output pattern (132; 240; 312; 412; 612; 712) formed on a lower surface of the light guide plate (130; 310; 520; 610; 820) and configured to output the second light to the lower surface of the light guide plate (130; 310; 520; 610; 820), wherein the first light source and the second light source are disposed on different side surfaces of the light guide plate that do not face each other; a reflector (140; 325; 625; 720) configured to reflect light output to the lower surface of the light guide plate through the second output pattern to the display panel; and a controller configured to control the light sources to output light for a 2D display mode or a 3D display mode. a first output pattern (131; 230; 311; 411; 611; 711) formed on an upper surface of the light guide plate (130; 310; 520; 610; 820) and configured to output the first light to a display panel (120; 330; 630); and a second output pattern (132; 240; 312; 412; 612; 712) formed on a lower surface of the light guide plate (130; 310; 520; 610; 820) and configured to output the second light to the lower surface of the light guide plate (130; 310; 520; 610; 820), wherein the first light source and the second light source are disposed on different side surfaces of the light guide plate that do not face each other; a reflector (140; 325; 625; 720) configured to reflect light output to the lower surface of the light guide plate through the second output pattern to the display panel; and a controller configured to control the light sources to output light for a 2D display mode or a 3D display mode.