Distillation is an old technique used to make alcohol, but also perfume, essential oil, or to purify water. This technique takes advantage of the different boiling points of different liquids. They will part from each other when heated. They will condensate into a liquid form when cooled down, at different times, and this is the moment the distiller will seize to collect them separately.
The distillate is usually collected in three moments starting with the foreshots (or the heads), followed by the heart and the tails (or feints). The different parts of the alcoholic liquid are called fractions. Each fraction has a different range of congeners. As ethanol is more and more concentrated the number of congeners falls, high strength spirits have less congeners and are lighter in character. Lower strength spirits have more congeners and are therefore heavier in character.
Most spirits are double distilled to achieve the desired alcohol level and some are even triple distilled such as Irish whisky or many many times distilled like some vodkas.
Depending on the type of spirit, the producer can either use a pot still or a column still to process to distillation, bother often made with copper.