Congeners are substances produced during the fermentation or distillation process other than ethanol. These substances include small amounts of chemicals such as methanol and other alcohols (known as fusel alcohols), acetone, acetaldehyde, esters, tannins, and aldehydes.
While many speculate on whether they contribute to hangovers or not, more importantly, congeners are responsible for many of the flavours in distilled alcoholic beverages and key to the distinctive character of a spirit. Brandy and Rum both have high amounts of congeners, while vodka has the least.
Type aside, some producers even test for the amount of congeners to make sure their spirit has a consistent flavour profile.