A bullwheel is a large wheel over which a rope turns. A bullwheel will be grooved all around its rim, allowing the rope or cable to fit in snugly without a risk of sliding off, and is generally the primary wheel attached to the drive shaft or other motive force moving the cable.
Bullwheels are used in chairlifts, cable cars, early farm implements, archaic oil wells, and other devices that used rope as part of their gearing. However, in the 1800s and early 1900s, the key characteristic of a bullwheel was often seen to be its size, so drive wheels and large gears might also be termed bullwheels -- although it is worth noting that a "bullwheel gear" is specifically the gear that will drive a rope or belt, as in a lathe.
Bullwheels are often paired with a calf wheel, which drives the drum that a rope or cable is wound around. While historically bullwheels were the larger of the two -- at least in diameter -- modern cable winders, still sometimes called, e.g., a 'bullwheel tensioner', often have bullwheels smaller than their calf wheels. When speaking of gears, it is much more common to refer to the larger gear as the bull gear and the smaller as a pinion gear.
The proper punctuation of bull-wheel is up in the air, and you will find the term used with a hyphen or space as often as not.