As others have explained, the terminology can be confusing. Drill, hammer drill, impact drill drill driver, drunk driver, whatever! So here's my slightly silly way of looking at it... You know the DRILL... The drill. Drill driver. You know this. This is what most people use or associate with making holes and turning fasteners. These use the traditional 10 or 13mm 3 jaw chucks, most decent models have clutches to tap out of the torque is too high for the application (like driving screws into drywall). Can make holes in wood, metals, plastic,ad nearly any material where just a spinny-spin motion is needed. Can fasten/unfasten most screws/bolts/nuts with the correct bit and as long as they are't too tight. Advantages: Great for making holes and some light fastener work. Cheapest option. Disadvantages: Limiting in other areas. Not torque enough to undo the wheel-nuts on your car. It's HAMMER Time The most common type of hammer drill is just called that... a hammer drill (or in confusing cases, an impact drill). Still has the 3 jaw chuck and does spinny-spins, but now with some added tappy-taps (when set to hammer mode). The taps are axial (or along the shaft of the drill bit). They hammer mode is almost primarily used for masonry (brick, concrete etc). But almost all hammer drills will have a regular drilling mode (and sometimes a fastener driving mode) so they are very versatile. Advantages: Versatile, not too pricey Disadvantages: Not great for serious masonry work. Still won't undo the wheel nuts on your car. For dedicated masonry work and when some extra brute force is needed, a rotary hammer drill is the tool. Bigger, beefier, and will pull you pants down if you use the belt hook. Most of these use a special SDS (or some SDS variant) chuck, requiring special slotted drill bits. Like so: They can do harder tappy-taps (in the axial direction) while doing spinny-spins, to get though things like concrete lintels and such. Some can also do tappy-taps without spinny-spins as a sort of chisel/jackhammer function. Not ideal for driving fasteners due to size, weight, and the need for an SDS-regular chuck (or some other) adaptor. Advantages: Powerful. SDS chuck system is great. an come with chisel functionality Disadvantages: Pricey, heavy, too specific, and not great for anything else really. Deep IMPACT Now we get into the actual question at hand... the impact driver. These use a quick-release hex type chuck. They are primarily used for driving fasteners and unlike the hammer drill, the impact action is tangential (perpendicular to the radius... damn that sounds even more confusing)... so basically thing of using a decent sized spanner on a tight bolt, stuck bolt. Then to get the bolt loose you hit the end of the spanner in an anti-clockwise motion with a hammer. That is how the impact driver do. When a certain amount of resistance to the rotation is met, it starts a hammer/impact action in in the same direction as the rotation. They can create massive amounts of torque (like I mean more torque than a mid sized car. The impact driver pictured above puts out 280Nm). One can also get drill bits to fit the hex-chuck for making holes in stuff, however there is no axial impact/hammer action so no good for masonry. The impact driver drill bits have the hex shank like so, and can be uses in a conventional chuck.: Advantages: more compact than a drill. great torque for for tough fasteners. Can drill holes (best suited to soft materials though). Hex chuck is super quick and easy to use. It also doesn't loosen itself or slip, like conventional chucks often do. And yeah... it will likely undo the wheel-nuts on your car Disadvantages: Not suited for masonry. Not ideal for all drilling applications. You filthy WRENCH The impact wrench is very similar to the impact driver, with the main difference being the "chuck", or lack thereof. Impact wrenches use conventional square drives (usually 1/2") to attach sockets for the driving of fasteners. They also have more torque and are generally bulkier. Some more powerful models (like the one below) are even bigger, but boast more torque than a supercar... we're talking over 1500Nm for the variant below) Advantages: very powerful. Great for driving big fasteners and tech screws into tough materials. Fits your socket set. Can undo the wheel nuts on your car Disadvantages: Too specialised for general use. Pricey. Will strip the wheel nuts off your car. Conclusion: Impact drivers are amazing. I use mine far more often than I use my drill. Mostly because I fasten/unfasten more than I make holes in things, and the size and hex chuck make it convenient. However if I could only have 1 of all the above, it would be a hammer drill as it's the most versatile. If you have a hammer drill, the addition of an impact driver will not go to waste. I'm fairly certain that like me, you'll find yourself using it more and more.