In this video, This Old House contributor Mark Clement explains the difference between a cordless drill/driver and an impact driver. An impact driver contains a different mechanism to drive the bit, so it can only use bits with a hex shank. An impact driver is both more compact and more powerful, but those advantages come at a cost: It's more expensive and considerably louder than a drill/driver.
Learn how to:
Differentiate between a cordless drill/driver and an impact driver
Understand the bit-driving mechanism
Evaluate whether an impact driver is right for your project