Cordless impact drivers use an impact force to increase torque when making holes and driving fasteners. The tool functions similar to a standard drill but has an impact mechanism that operates when the tool encounters resistance. The impact force is similar to a hammer hitting an anvil while the bit or fastener is rotated between impacts. Impact drivers are typically lighter and smaller than standard drills and impact wrenches. They have a 1/4" hex head that accepts 1/4" hex impact-rated bits. Cordless tools run on batteries, so they are portable and don't require access to a wired power source. Brand-specific batteries can often be used with various cordless tools of the same brand/manufacturer that have the same battery series and voltage.