How is a driver different from a drill? What

Hello, W/D here.

A very good question......A drill holds a bit and rotates it at a given speed. This speed can be variable, allowing the speed of the drill bit to be better matched to the material being drilled. The output from the drill motor goes directly to the chuck, and the power is directly applied to the drill bit. Some drills can generate a tremendous amount of direct torque, due to the nature of their gearing. Most of the better drills have planetary gears in them.
A driver rotates like a drill, but is designed to apply torque to a driving bit, and cause a fastener to be placed by the driver. The main difference between a drill and a driver is that a driver has an adjustable clutch, allowing the amount of torque being applied to a fitting to be preset. A good example of this would be for driving wood screws. You would dial in the torque setting that you want the driver to quit driving the screw. You don't want to drive the screw to China, you want to drive it flush. A maximum torque setting "locks" the clutch, and the fitting will be driven as far as it can go (This is about as close to being called a drill as a driver will ever get). A clutch setting midway might be just right for driving the same fitting into oak, and a setting at less than that might be just right for pine. The torque clutch effectively sets a kick out torque for the driver. When the torque applied matches the torque set on the driver, the clutch "slips", and no further driving action can occur.
Most modern battery powered drills incorporate a torque clutch between the motor and the chuck so that the tool can be operated as a drill (with the torque setting at "max") or as a driver (with the torque setting at less than max) some electric tools are configured as both, but usually they are different. For the money, a good battery powered drill/driver with a clutch offers more versatility, in my opinion.
Best regards, --W/D--

Screwguns are more accurate in setting screws to the correct depth. This is particularly desirable when installing dry wall. You want the screw to be indented but not break the paper. Drills that have clutches can perform very well ,once the clutch is set right. Drywall screw guns run at high rpm's, about 4000rpm's, which works well when installing drywall on metal studs. Deck screwguns run at about 2500rpm's,but they have a good deal of torque to drive longer length screws.
Drills are much more versatile than screwguns. There is even a dimpler attachment that is used for setting screws in drywall, and it works. So, unless you have lots of drywall to hang, or you're building lots of decks, I would recommend a good quality drill. I've used a slower speed drill with lots of torque for decks, and I've used a higher RPM drill for hanging drywall on metal studs. Both worked, and of course the drill has so many different uses.