Trigger: The power button. The more you press on variable-speed triggers (which all the winning drills have), the more power is exerted.
Forward/Reverse Selector: Lets you choose forward to drive in a screw, reverse to draw it out.
Chuck: The adjustable jaw that clamps or releases the drill bit or screw tip.
Clutch: Makes the drive shaft stop turning when it meets a certain level of resistance from the screw (which you preset) to keep you from stripping or overtightening it or putting it in deeper than you want. Figuring out the exact setting for a job is a trial-and-error process, but you should generally use the lower settings with softer materials like drywall or short screws; the medium settings for wallboard, pine, and brick; and the highest for cement, metal, and dense woods. The drill position (usually marked as an image of a drill bit) is for pilot holes and other drilling jobs.