What's the difference between a drill/driver vs. an impact driver?
Cordless drills have become a mainstay in the construction industry, and have long been the most wonderful thing not only for drilling holes, but also driving screws.
Along comes the newcomer, the cordless impact driver, and it has become the favorite for driving screws and lag bolts.
Impact drivers have a lot more torque than a drill/driver, which in and of itself really helps for driving screws. But they have also added a mechanism referred to as the "hammer and anvil" that really adds oomph to driving in screws.....and does so without grinding out the head of the screw as drill/drivers so often do.
Both tools have their place. Drills are still better in general at drilling holes. Impact drivers are generally better at driving screws, lag bolts, etc. There are always exceptions to this but that will hopefully give you a better idea as to the roles of both tools.