We deal with two types here: Mains and Cordless. For additional details on drill-bits and their usage see the Drill bits and Drilling FAQ.
Probably most peoples first DIY tool purchase. Essential for the classic DIY task of affixing shelves, but also able to make holes in most materials, sand down (quick and rough), grind, polish etc… if you start looking at all the add-on gadgets you can get, a drill becomes a very versatile bit of kit.
Drills in their most basic form are single speed with rotation in one direction only. These are fine for drilling in wood, and also OK for many polishing, sanding and grinding operations. They are also quite small and hence can be handy for getting into tight spaces, like between joists, etc.
Adding things like variable speed and reverse expand the range of tasks that can be done safely, such as screw-driving, and drilling metals. The addition of a gearbox with two or three speed ranges also adds the ability to use less speed and get more torque for tasks that will benefit. The other usual addition is “hammer” action. This gives you the ability to drill hard stuff like masonry.
For drilling big, or deep holes in masonry, especially really hard materials like concrete or engineering bricks, the SDS drill now available on the DIY market, will trounce any hammer drill, as well as adding some party tricks of its own. First-time users of SDS drills often quote a phrase like “hot knife through butter” when searching for words to describe just how much better they are.
The bigger, more powerful, drills can turn tools like large hole saws, core borers (for wide diameter holes in masonry), and are good at mixing stuff with a suitable mixing paddle.
For basic operations, the budget tools will do pretty much what the high end ones will, though the low end ones often don't last very well. Spending more money will buy you better endurance from the motor, so you can run it longer without rest periods, better speed controllers, and more robust gearboxes. Bearings will improve and become more impervious to dust - handy if you do much masonry work, or lots of grinding and sanding. If looked after, even a basic drill should last years.
The cordless drill is a godsend any time you need a drill and the freedom from a mains flex. It is ideal for screw driving, where the DC motor will provide a much better control of power than most mains drills. If you assemble flat pack furniture, then a cordless drill will save many hours of work!
The spread in performance between budget and high end is very marked in cordless drills, far more so than with mains drills. The cheaper end of the market can be pretty disappointing – to the extent that it is often better looking only at mid-range tools or better. Remember that a good amount of money will need to be spent on batteries and a charger before you are can get decent performance.
Two types are readily available, the Drill/Driver and the Combi Drill, which adds a hammer action. The former will be cheaper and in many cases more than adequate if backed up by a mains or SDS drill for times that hammer is needed.
All will have reverse, and most will have a speed controller, except the lowest voltage types, e.g. 2.4V, as on these it would not gain you anything. Speed control is essential for serious work, and when implemented well, will greatly enhances the usability of the tool. Good speed controllers can deliver slow rotation while providing plenty of torque (turning force), and this is ideal for some screw-driving tasks. Some better tools implement a rotor brake that stops the rotation when you release the trigger. This helps to avoid accidentally driving a screw too far into the work, stripping threads etc.
Many have a variable torque limiter, which allows you to set how much to tighten a screw. It can make the task of putting in lots of small screws quick and easy, since you can be quite ham fisted with the trigger in the knowledge that the drill will back off before you over do it! With better tools the repeatability of the limiter improves.
Having more than one battery is very much recommended. If you have three and a good charger, then chances are, you can keep going all day, and you will be worn out long before it is!
If you are talking about a good quality tool with decent cells then the limits of performance are roughly:
2.4/3.6V: These are basically electric screwdrivers, not drills. They are good for light screw-driving and flat-pack assembly, but you typically need to do the final tightening by hand since they run out of puff. They will only drill a few holes in wood before going flat, or drive a couple of dozen 2" screws home (and then only with a pilot hole). They will cope with all but the largest appliance screws. No use for heavier work.
7.2V: These would typically have a two-speed gearbox, and no speed controller. They are suitable for frequent light screwing and drilling tasks, very good for assembling flat-pack furniture, and OK for drilling in wood. The gearbox may not be that robust, (one report speaks of a tooth stripped from a gearwheel when a spade-bit snagged). The performance limit of such a tool is typically driving a 2" screw into a Rawlplug, or about 1.5" into solid timber (if not pilot-drilled).
9V: This drill will do most wood drilling tasks, but will struggle with larger spade bits. Hammer action (if available at all) will be feeble but better than none. Screw driving will start to have difficulties with 4” and bigger screws into softwood.
12V: This will get a 4” screw driven home with more authority, and have a better performance on masonry.
14.4V: This will deal with pretty much any screw, handle smaller hole saws, and have a reasonable hammer action.
18V+: These will swing a 5” hole saw, mix a bucket of plaster, and stick a 6” roofing screw into solid wood without any difficulty. It is at this level you match the power of a smallish mains drill, but with far more finesse and controllability. However the weight and size is creeping up so it pays to choose one with a nice balance to it.
24V: These are specialist and powerful tools, heavy in weight and price.
If you are looking at a £29.95 18V combi drill special offer on the back of a catalogue then all bets are off, but it might make a nice dumbbell!
The chuck in a fundamental part of a drill and yet often overlooked. Common shank diameter capacities are 10mm and 13mm (3/8" and 1/2"), with the latter being well worth having since the shanks of some tools are too big for the smaller one (many mixing paddles for example required a 13mm chuck). Keyless chucks have become popular recently. Good ones (like those made by Rhom) incorporate a ratchet type tightening action that allow them to be tightened at least as well or better than conventional chucks that require chuck key. The poorer keyless chucks are not as good as a keyed one in ultimate gripping power, but are still adequate for most jobs.