Ancient Greek is a useful guide, since it has at least 4 words which translate into English as love.
Storge - this comes from the word meaning 'womb' and describes the mother-child love, and also a whole range of situations of varying intensity in which we might use the word 'affection', such as love for our annoying older brother or our untalkative great aunt. It can attatch to any familiar face.
Philia - this is roughly 'friendship' and, in the Classical world, was believed to be the finest of the loves. It revolves around some shared interest or experience.
Eros - this is the element of love distinct to a 'romantic' relationship, it is more lofty than mere sexual attraction.
Apage - this is the word usually used to describe God's love. It is unconditional and so not a response to any lovable quality in the beloved; for this reason it is completely selfless and the most permenant of the loves since no change in the beloved will reduced the intensity of this love.
The classic, and highly recomended, study of these attributes of love is C.S.Lewis: The Four Loves [order from amazon.com].
Lewis explains that the 4 loves are not exclusive, but rather many relationship will contain elements of a couple of them, and a healthy marriage really needs all four. He also categories love in another, fairly self-explanatory, way into Need-love, Gift-love and Appreciative love. Again, most relationships will contain a combinaton of these.