Before the mid-20th century, typologies of love were works of philosophy, ethics, introspection, and intuition. In the 1980s, Robert Sternberg produced a typology of love psychometrically, meaning he asked people about their experiences and used factor analysis to determine which experiences tended to co-occur. He came up with a three-factor model of love: intimacy, passion, and commitment. Intimacy is stuff like warmth, closeness, and bondedness. Passion is stuff like romance, physical attraction, and sex. Commitment is the decisions involved in maintaining love over time. By combining those factors, he came up with the following typology:

Relationship Type Intimacy Passion Commitment
Nonlove Low Low Low
Liking High Low Low
Infatuation Low High Low
Empty Love Low Low High
Romantic Love High High Low
Companionate Love High Low High
Fatuous Love Low High High
Consummate Love High High High

Here’s a typical triangular image of the system: