In C.S. Lewis' book The Four Loves Lewis discusses eros in the third chapter of his book. He defines eros as the state of being in love. To me he makes it sound sort of like a crush. It is not lust for someone though; it is the passion of a relationship. The new found love that two newly weds feel for one another.
Lewis goes through each of the four loves in four different chapters. He emphasizes that we need to have all four of these loves to have a lasting relationship with someone else. Storge is the type of love that a mother and her child share. It is the affection that two people have for each other; the caring a mother feels for her son or daughter.
Philia could be best described as friendship. Liking another person and being able to have fun with them. This is a very important component to a relationship. People can care for each other, but if two people never have fun together there relationship will not last.
The most important part of a relationship is agape. The love of God that brings to people together. This is why it is so important to be a mutually christian relationship, so both members of the relationship move closer to each other as they move closer to God.