A wadcutter is a type of bullet. Intended originally for use in target shooting, the wadcutter was designed to punch neat holes in paper (wadding) to allow for easy and precise scoring of the shot. As a result, the wadcutter bullet is flat-nosed, with the nose very nearly as wide as the cartridge itself, making the bullet almost cylindrical. They are used in either revolver action weapons or in single-shot rifles, since the lack of an angled or curved bullet means the cartridge won't properly seat itself in magazine-fed weapons. Some wadcutters are made with the bullet resting entirely within the cartridge, presenting a completely flat top.
In addition to target use, they are used for self defense and hunting when the purpose is to maximize lethality and stopping power. The sharp edges of the bullet cut large holes through the target, increasing the wound lethality. Compared to hollow-point or soft-nose bullets, they will penetrate farther without deforming since the front of the bullet is larger and stronger - this is important for rapidly disabling the target, be it a person or game. The aerodynamics of the round, however, make it inaccurate at long distances and cause it to rapidly lose energy, so it is used at close ranges.