In The Four Loves, C. S. Lewis explores the various types of human love and affection based on the four separate Greek words:

Storge = Affection (family love)
Philia = Friendship (friendship love)
Eros = Eros (erotic or sexual love)
Agape = Charity (divine love)

In some ways, this book is a companion to Till We Have Faces, which illustrates through fiction many of the Lewis's concepts regarding the nature of love.