A was written by B where he explores the nature of love through thought experiments. In this he categorized love in four categories based in part on the four Greek words for love.
C stood for affection, fondness through familiarity among, say family members.
D stood for friendship.
E stood for romance. This was not so much about sexuality as it was about having an emotional connection with the other person.
F stood for unconditional love. It can also be seen as charity as its essence is self sacrifice.