Single-shot firearms are guns that only hold a single round of ammo. After this round has been used, the weapon must be reloaded. All firearms have evolved from the Single-shot. Single shots are still manufactured, however they are now in both cartridge and non cartridge variants.
In the early days of cartridges, Rifles were single shot, taking great use to single-shot motives. The earliest cartridge rifles were conversions of muzzle loading rifles. With these, you would mill out the rear of the barrel, and attaching a folding bolt that flipped up to the cartridge.
An example of these is the Springfield 1873 rifle.
Single-shot pistols were not as common as rifles were. This is because revolvers were a matures piece of technology one cartridges have arrived. single-shot versions though did exist. These fell into two groups- derringers and target pistols. Targets were pretty much rifles in pistol size.
Single-shot shotguns have been quite a popular item since their invention. This is partially because they have been significantly less expensive than double barrel variants. these are widely used in activities such as hunting.