Impact driver, I though it was called an impact wrench.  While these share many similarities, they are different.  This article talks about Impact Drivers.  Before we start we need to first understand what an impact driver is.  An impact driver is simliar to a drill, but as the name suggests, it also impacts while rotating.  Lets say you are driving a screw into a piece of oak wood.  When you start drilling the screw, the impact driver works like a drill and rotates screwing in the screw.  You will get to a point where the pressure build around the screw will require more force to drive home the screw and set it.  The impact driver will sense this resistance and start its impact procedure to help provide more torque and pounding to get the screw to set.

Behind the bit holder is an anvil, which is a two lugged gear.  A drive gear with two matching lugs will repeatedly strike the lugs of the anvil causing an impact.  This extra force helps produce more power to your application than straight drilling. 

You may be thinking, my drill has a hammer feature which is similar, right?  Well not exactly.  A hammer drill is a lot different than an impact driver.  While they both rotate to drill, the similarities end their.  A hammer drill or the hammer feature that is on your drill has one primary function, and that is to drill through Concrete, rock or other hard material.  You should not use the hammer feature when trying to drill into wood or other like materials.  When drilling into wood, the impact driver is made for this and performs great. 

All most all the major manufacturers now make an impact driver.  These are getting so popluar because of their ease that manufacturers are inlcuding them in their kits.  Most manufacturers make different volts of Impact drivers and you can find them from 7.2V and up.  So depending upon what you will be using the impact driver for, you can find the right size for your needs.