Stills do not make alcohol, but concentrate it instead. Because of this, the beginning of whiskey distillation begins not with the still, but with the fermentation that precedes it. One of the key elements separating different types of whiskeys is the recipe of grains used in creating a mash for fermenting and the production of base alcohol. For example, bourbon is made mostly with corn, while single malt scotch is made entirely from barley. Without this start, there would be no alcohol for a whiskey still to distill in the first place.
Whiskey distillation takes advantage of the fact that alcohol has a lower boiling point than water. At sea level, water boils at 212 degrees F, while alcohol boils at 172 degrees F. Therefore, if the fluid from the finished grain mash is placed in a pot and heated to between 172º F and 212º F, the alcohol will boil away and leave the water and other substances behind. The vapor rises out of the boiling pot and into either a coil or a second pot, where it is cooled and condensed into a more concentrated alcohol liquid.
Classic whiskey stills are made of copper, because of the metal's even heating properties. They are built using the simple pot still design, which uses two connected vessels, one for boiling and one for condensing. However, some whiskeys are made using alcohol that has been distilled multiple times, and these sometimes use a reflux still. In this still, the boiling pot is built as a column lined with glass beads, which increase the surface area of the boiling pot. The alcohol condenses and boils off the beads multiple times as it makes its way up the column, increasing distillation.