In medieval times, romance was guided by the concept of "courtly love." Courtly love was characterized by being highly stylized and not very realistic. However, it did have well-defined rules and categories. One of the better and more concise breakdowns is as follows (See link below for more information and citations):

Aristocratic: As its name implies, courtly love was practiced by noble lords and ladies; its proper milieu was the royal palace or court.

Ritualistic: Couples engaged in a courtly relationship conventionally exchanged gifts and tokens of their affair. The lady was wooed according to elaborate conventions of etiquette (cf. "courtship" and "courtesy") and was the constant recipient of songs, poems, bouquets, sweet favors, and ceremonial gestures. For all these gentle and painstaking attentions on the part of her lover, she need only return a short hint of approval, a mere shadow of affection.

Secret: Courtly lovers were pledged to strict secrecy. The foundation for their affair--indeed the source of its special aura and electricity--was that the rest of the world (except for a few confidantes or go-betweens) was excluded. In effect, the lovers composed a universe unto themselves--a special world with its own places (e.g., the secret rendezvous), rules, codes, and commandments.
Adulterous.

"Fine love"--almost by definition--was extramarital. Indeed one of its principle attractions was that it offered an escape from the dull routines and boring confinements of noble marriage (which was typically little more than a political or economic alliance for the purpose of producing royal offspring).

Literary: Before it established itself as a popular real-life activity, courtly love first gained attention as a subject and theme in imaginative literature. Ardent knights, that is to say, and their passionately adored ladies were already popular figures in song and fable before they began spawning a host of real-life imitators in the palace halls and boudoirs of medieval Europe.