The first of the 'self sparking' lock types to be developed, the wheellock relied on a spring powered clockwork mechanism to drive a notched steel wheel against a block of iron pyrite, thus generating a jet of sparks to fire the powder and shot ammunition. Developed around 1500 it co-existed with the matchlock until about 1600 when both began to be replaced by the flintlock mechanism.
If this all sounds a little bit … complicated, it was. It stretched the manufacturing technology of the time and was expensive, fragile, prone to breakdown, greedy for maintenance and required pre-winding with a key that was often hard to replace if you lost it. It was also a significant improvement over the matchlock that preceeded it because it "made its own fire" and the user didn't have to carry a firepot or burning slow match around next to his black powder. Besides not being constantly at risk of being blown up, this also meant that he could conceal himself and his weapon without a burning smell and cloud of smoke giving them away.
This fact alone provoked a significant outbreak of hoplophobia - and plenty of entirely legitimate fear as well - amongst the great and allegedly good of Europe as their worlds suddenly filled with the prospect of stealthy men with firearms leaping out at them from dark corners. Some magnates, including at least one Prince of Orange, were, in fact assasinated with concealed wheellocks and various laws were passed promising dire penalties for possession1 of such weapons … most of them not quite as dire as the penalties for even the attempted treason and/or regicide that they were meant to avert2. Such laws lasted for a while and then, like the crossbow panic before them, the powerful went on to worry about other things.
Because of its mechanical complexity the wheellock never really replaced matchlock weapons, but it created the market for self-sparking triggers that would eventually lead to both being replaced by the flintlock.
Largely a technology for small arms as it was an unecessary extravagance for ordnance - which remained more or less happy with cannon lock - and exclusively a technology for black powder muzzle loading weapons, since it was obsolete long before anything else came on the scene.