Why Cordless?
Charge it up and take it anywhere. With a cordless drill, you can work anywhere. Cordless is easier to use in confined spaces and especially useful in the garden if you have no access to a powerpoint. Working outside in the rain, it's way more preferable to having a mains voltage extension cord under your feet.
Drills Dewalt
Most cordless drills nowadays feature a 'keyless chuck'. This allows an easy change of drill bits with no tools required.
Drill Speed
Drill Speed is measured in rotations per minute (rpm). With a variable speed cordless drill you also have a powerful cordless screwdriver in your toolset. Start driving slowly and then speed up once the screw 'bites'. Switch into reverse to loosen stubborn woodscrews - much easier on the arm than a normal screwdriver.
Hammer Action
With a hammer action drill, the chuck moves backwards and forwards as it spins - hammering as it drills. This makes drilling into brickwork and masonry easier.
Voltage
The higher the voltage the more power you have at your disposal. A high voltage drill will also require a larger battery that in turn will make the drill larger and heavier. If you intend to do mostly masonry drilling then buy the highest powered drill that you can sensibly afford. Cordless drill voltages commonly range between 12-24 volts.Typically, a 12v or 14.4v drill will be fine for occasional use while an 18v or 24v drill would be better for a professional tradesman.
Battery Type
Typically, at the moment, most cordless drills are sold with NiCad or NiMH rechargeable batteries. The latest generation of batteries being used in some cordless power tools are the Lithium-ion (Li-ion) batteries. These are lighter, run longer and maintain their charge much more effectively than previous rechargeable batteries.
Charge Times
Battery charge times vary from model to model. Some drill/charger combinations feature 'fast charge' settings. If your drill is intended for sustained use, get a spare battery. Keep one on charge while using the other.