Choosing the best voltage for your drill is a perennial question, and you have basically 4 choices of voltage to consider in the world of drills.
4-8 volts – A voltage rating of 4 to 8 volts are sufficient for light-duty cordless screwdrivers that you use around the house to drill many light projects and low voltage drills and screwdrivers are very inexpensive. Generally, you can pick up a cordless 4-8 volt drill or screwdriver for a great price.
12 volts – Considered the best for normal home use, DIY projects and light work. 12 volt drills generally have a 3/8 inch chuck. With a 12 volt cordless drill you shouldn't have a problem with 3-inch deck screws or boring holes with sharp spade-bits. While 12 volt drills are great for light use, they are not the best drills to use with hole saws or larger auger bits.
18 to 20 volts – If your job is going to be on a larger scale and require more intensity - especially for remodeling jobs, you'd be better off with 18-20 volt drills. They generally have a 1/2 inch chuck. Makita makes an excellent 18 volt cordless drill that has 480 lbs of torque and comes with a contractor grade set of drill bits compatible with 1/4" impact drivers and 3/8" or 1/2" driver/drills.
28 volts and onward – 28 volt drills are best for professional contractors who have a lot to drill, ie concrete, metals studs, joists and more intense materials. Milwaukee's 28 volt hammer drill is an excellent choice for contractors who need a lot of torque. Milwaukee's hammer drill has 750 lbs of torque and will let you drill through pretty much anything on the job site.
If you are only buying one drill and most of your work will be done at home on DIY projects or light home remodeling, get an 18-20 volt model.
You'll have enough power to do most DIY jobs around the house or projects that need larger bits.
If you are a contractor with a variety of heavy-duty materials to drill into (like masonry or joists), a 28 volt drill is your best choice, but you'll probably need to buy a second 18-20 volt drill to handle the more basic contractor projects like hanging cabinets, installing doors and decks.
If you have a home DIY reno project or need to use a hole saw to drill through doors or use large boring bits, the 18-20 volt drill is a good choice. It will give you a lot of versatility and be a great home renovation drill - especially if you buy a cordless model like the Makita 18 volt cordless hammer driver drill.
12 volt drills are good for most light duty home projects and will give you enough power to make home repairs easily. But, they are not good for much else that requires more torque. Have a new set of curtains to hang for the wife? A cordless 12 volt drill will work well and not drill a hole through your wall, unless you screw up.
4-8 volt drills are pretty basic and have very little torque - think of them as a screwdriver replacement. It will save you the hassle of using a screwdriver for little projects.
Ultimately, we recommend the more robust 18-20 volt drill for use at home and as a complimentary drill for a contractor who has a 28 volt drill with a lot of torque*.
Power Drill Definitions:
Torque - The rotational force or turning force of an object. If you have very little torque, as in a 4-8 volt drill, you won't be able to drill into materials heavier than drywall or pine wood. The turning force of the 4 volt drill is very weak - imagine the strength of a 10 year old or so trying to apply pressure to turn something. However, if you buy a 28 volt drill, you'll have 750 lbs of turning force - which can be imagined as having a few good men applying pressure to turn the drill. Every time you step up in voltage you'll get more torque in your drill.
Volt - One volt is the difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points. Basically, a volt is the amount of power or current flowing between 2 points. The more volts flowing, the more power you'll have.
Chuck - The chuck is the part of the drill that holds the drill bit. It generally has a hexagonal shape and most power drill bits fit most drill chucks. If you are using a smaller drill, like the 4 volt type, the chuck will be a lot smaller because the drill bits are lighter in weight. If you are using a larger drill, like the 28 volt type, the chuck should be 1/2 inch to hold the more robust drill bits. If you want the maximum flexibility in a power drill, buy one that has a 1/2 inch chuck.
Cordless vs Corded Drills - A cordless drill is one that uses a rechargeable battery. The convenience of cordless drills can't be overstated, but the trade off between a cordless drill and a corded drill is weight. The base of a cordless drill generally is the removable battery which can weigh quite a bit (6lbs or more). When buying a cordless drill look at the weight of the drill and also how many batteries will come with your purchase. A corded drill will weigh less, but you'll need to have a power source available and plenty of extension cords. Corded drills are good for home use, but if you are a contractor or working on a project away from a power source, you'll need a cordless drill.