A disc wheel is a wheel with solid sides, instead of spokes. Although disc wheels are heavier than spoked wheels, they have less air resistance, and permit a cyclist to go noticeably faster. They are not usually used on the front, because they can cause severe handling problems in cross winds. They are not usually used for hilly conditions, because the added weight is too much of a disadvantage on the climbs.
The illustration is from Archibald Sharp's Bicyles and Tricycles, originally published in 1896, reprinted by the M.I.T. Press in 1977.
The front wheel has cutouts to reduce steering problems that result from crosswinds when disc wheels are used on the front.