A sour is essentially a simplified Punch, consisting of usually just three ingredients:
The sweetener can take the form of sugar or simple syrup, or it can be a liqueur of some sort which has a prominently sweet flavor.
The sour is usually either lemon or lime juice, but can technically be any high-acid ingredient which would lend the same character.
While at one time considered a mixed drink category all of it's own, in more recent times it appears to be seen as a sub-category of the Cocktail. Some common sour style drinks are: