I have some experience with 2 of the 3, but not a greater understanding. Using a drill/driver and an impact driver, I prefer the impact for any 1/4” hex bits and save the drill for actual drill use. Am I doing that right? Secondly, what is the difference between the drill and a hammer drill? My frame of reference is looking into picking a cordless tool system.
Sounds about right.
1/4" Hex impact for driving screws or lag bolts
3/8" square impact for automotive stuff/machine screws.
Regular drill for drilling in wood, metal, plastic, etc.
Hammer drill for drilling in concrete, brick, etc.
Most drills are combo drill these days and do hammer or non-hammer drilling.
Hammer drills are good for small holes in masonry, tile, etc, but if you want to put decent size holes into concrete, stone, etc, look into a rotary impact. Many hardware stores will rent them. They're usually also good for light jack hammering type operations, like stripping tile off a concrete floor.
Impact drivers are great for driving threaded fasteners. You can use a drill for that, but you are at the mercy of the torque of the motor. An impact driver "throws weight" instead of using pure torque like a drill. An impact driver is a smaller, electric version of a pneumatic impact wrench.
Drills make holes and do spinny things.
Hammer drills are drills that add longitudinal motion. They don't impact like a driver, they still do spinny things with torque, but they add an in/out motion to the drill bit. This is almost exclusively for drilling masonry. In addition to a spinning drill bit, it whacks the tip of the bit against the masonry to chip out bits and make it easier to drill.
Drill is torque
Impact driver is horsepower
Hammer drill is torque plus hammer
RX is referring to a Rotary Hammer, which is like a hammer drill on steroids. They are half way between a hammer drill and a jackhammer. They usually have a gear box that lets you select just drilling, hammer drilling, and just hammering. They have special SDS chucks for bits and you can get masonry drill bits, chisels, points, etc for light jackhammering.
Take note in this photo of the selecting thing on the side. This is like the one I have. You can use it as a monster drill, a hammer drill, or a mini jackhammer.
So for general use a hammer drill isn’t needed.
In reply to Brian :
A hammer drill doesn’t work on metal or wood.
Because it pushes the bit in and out (hammers), it would repeatedly disengage a metal cutting bit. That would ruin the cut, and destroy the bit.
Hammer drill for masonry only.
Brian said:So for general use a hammer drill isn’t needed.
No, but for all the difference in cost, might as well get a combo so you have it if you need it.
Let me reword that....
Hammer drills have settings on them that can turn off the hammering.
You CAN use a hammer drill for metal, but you have to turn off the hammering feature and use it as a standard drill.
I wouldn’t want a hammer drill for basic drilling all the time. They are heavier, and bulkier to use.
Small hammer drills (like the cordless ones) are not overly heavy, but they are always heavier than their basic drill brothers of similar size.
All depends on what your needs are.
Having a regular drill and an impact driver is the way to go if you're dealing with wood and metal. The hammer drill in non-commercial applications is a bit of a marketing ploy.
It's helpful to have the drill to make pilot holes, and then the impact to drive the fastener home. Plus, it guarantees that you'll have two batteries, which is always a plus!
If your place of living has concrete walls or floor, you might as well spend the extra couple dollars for the hammer function. You may wish to tapcon something one day.
I have a small Makita rotary hammer what takes SDS-Plus bits. It's the Shiny Happy Bob Costas! 
dj06482 has it right.
I keep my cordless drill and impact driver in the same case. I use them all the time.
And although I am in commercial construction, my hammer drill stays in the case and is rarely used.
I’m looking at the HF Bauer line and full price the hammer is $5 more than the regular drill but there are some nice coupons out this month on the drill one some others but not the hammer.
Although being HF, everything becomes a hammer.
Have a 3/8" Milwaukee selectable drill/ hammer. Somewhat of a compromise, it's been good for around home work but I wouldn't call it commercial. It lost some of it's 'hammer' although it's not a hammer in HF terms. Great drill btw.
There's nothing like a Hilti
Hit up your local Home Depot tool rental if you need a hammer drill. They sell them after not too much use. The Makita I pictured above is the same one I bought from HD a few years ago when I needed to cut a hole in concrete block for a PVC exhaust for a furnace. It has proven very handy. Having worked at HD tool rental for a few years, you can trust their used tools. They are impeccably maintained and rarely fail. When they do fail, they get factory parts and get repaired quickly so they can get back on the shelf for making money.
Get the smaller one. They use SDS plus bits which are commonly available at HD, Lowes, and HF. If you get the larger SDS-Max, those bits are usually pro-only and pretty pricey. For a home improvement tool, SDS plus should do more than you need. You can usually get concrete hole saws for SDS plus in sizes up to 1.75". If you need to drill for 3" or 4", you'll have to go SDS-Max. I have seen hole saws up to 4" for SDS plus, but they won't perform well at all.
I have two drill-driver kits. One is an earlier Ridgid drill/driver combo, and the other is a later Hammer drill/driver combo. So in my arsenal I have all I need: Two impact drivers, one cordless drill, one cordless hammer drill for small masonry jobs, (or turn the hammer off and its a regular drill) a corded drill for heavier stuff like drilling metals, and the SDS hammer drill for larger masonry projects.
I find myself rarely using the corded drill. My cordless will drill steel just fine, but heavier steel projects sometimes means going through a couple charges. They have plenty of torque, just not the battery life for doing a lot of holes in heavier stuff.
If you're just drilling some 1/4" holes in concrete to hang some shelves, any hammer drill will do; cordless, corded, whatever. If you have a bigger project and need a bunch of 3/8" holes, or a 2" passageway for conduit, step up to SDS
The way a cheap hammer drill works is that you put the bit in a traditional chuck, then the hammer function punches on the chuck. It isn't a very heavy-duty way for a bit to hammer on masonry. An SDS has keyed flutes on it that allow the bit to float in the chuck and the drill then hammers on the bit itself. Much beefier and more reliable. Small projects can use a chuck-style hammer drill. Bigger stuff or long-term use should use SDS
In reply to Brian :
I bought that baur half inch cordless drill/hammer drill a few months back and it's been great so far. Better feel than the rigid it replaced, bigger chuck, but a slightly more annoying light. Reminds me a lot of the makita cordless tools. The battery life has been surprisingly good so far as well. Pickup the extended warranty and take it in for a no questions asked replacement before it expires if you're worried or start having problems.
Truthfully, I prefer my hammer drill to plug-in, which is why I have a rigid plug-in model as well. It's considerably larger and bulkier, but meets my needs when I need it. The 30 year old black and Decker it replaced was smaller, lighter, and considerably weaker even before it pulled a Porsche impression and lit itself on fire while I was using it.
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