Similar to a long finish, balance is a sign of quality. A wine is balanced when acidity, tannin, alcohol and sweetness are in harmony; none of the components overpower the others.
These wines are described as balanced and harmonious.
Tannin and acidity are "hard" components. They make the wine firmer and provide structure to wine, much like the steel girders in a building or the skeleton in a human body.
Wines with good levels of tannin and acidity can be described as structured or firm -- and may also be called masculine.
Alcohol and sweetness are the "soft" components. They make the wine softer, rounder, and prettier, much like flesh on a human...