What is love? It is one of the most difficult questions for the mankind. But no one can give the proper definition of love. To some Love is friendship set on fire for others maybe love is like luck. You have to go all the way to find it.

Martin Luther King once said, “Power without love is reckless and abusive, and love without power is sentimental and anemic.”

Psychology portrays love as a cognitive phenomenon with a social cause. It is said to have three components in the book of psychology: Intimacy, Commitment, and Passion. Also, in an ancient proverb love is defined as a high form of tolerance. And this view has been accepted and advocated by both philosophers and scholars. Love also includes compatibility.
Source: Love-sessions.com

Ancient Greek has four distinct words for love.

• Agápe (αγάπη agápē) - Charity - A duty bound love that does what is in the best interest of the person loved. An unconditional love. In Ancient Greek, it often refers to a general affection or deeper sense of "true love" rather than the attraction suggested by "eros". Agape is used in the biblical passage known as the "love chapter", 1 Corinthians 13, and is described there and throughout the New Testament as sacrificial love. Agape is also used in ancient texts to denote feelings for a good meal, one's children, and the feelings for a spouse. It can be described as the feeling of being content or holding one in high regard.
  
  The term s'agapo (Σ'αγαπώ), means "I love you"
• Philia (φιλία philía) means friendship in modern Greek. It is a dispassionate virtuous love, a concept developed by Aristotle. It includes loyalty to friends, family, and community, and requires virtue, equality and familiarity. In ancient texts, philos denoted a general type of love, used for love between family, between friends, a desire or enjoyment of an activity, as well as between lovers. Philadelphia, the city of brotherly love, gets its name from this.
• Éros (έρως érōs) - Romantic - is passionate love, with sensual desire and longing. The Modern Greek word "erotas" means "(romantic) love;" however, eros does not have to be sexual in nature. C. S. Lewis calls sexuality "Venus" after the goddess of love and beauty. Eros can be interpreted as a love for someone whom you love more than the philia, love of friendship. It can also apply to dating relationships as well as marriage. Plato refined his own definition: Although eros is initially felt for a person, with contemplation it becomes an appreciation of the beauty within that person, or even becomes appreciation of beauty itself. It should be noted Plato does not talk of physical attraction as a necessary part of love, hence the use of the word platonic to mean, "without physical attraction." Plato also said eros helps the soul recall knowledge of beauty, and contributes to an understanding of spiritual truth. Lovers and philosophers are all inspired to seek truth by eros. The most famous ancient work on the subject of eros is Plato's Symposium, which is a discussion among the students of Socrates on the nature of eros.
  Éros does not appear in the Bible
• Storge (στοργή storgē) means "affection" in ancient and modern Greek. It is natural affection, like that felt by parents for offspring. Rarely used in ancient works, and then almost exclusively as a descriptor of relationships within the family. It is also known to express mere acceptance or putting up with situations, as in "loving" the tyrant.

I use the acronym APES (Agápe, Philia, Éros, Éros) to remember them.

The Bible:
Love is considered by many to be the dominant theme of the New Testament of the Bible.
In response to the question "which is the Greatest Commandment?" in Mark 12:28-31 and Matt 22:35-40, Jesus replied:
"'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself'." Also Leviticus 19:18.

The New Testament in the Bible was originally written in Greek and includes all four of the meanings above. Christians have used them to how they might apply to todays relationships between people and in a marriage.

The book of John is called "The Gospel of Love," is about a personal relationship with Christ/God.
1 Corinthians 13 is the Love Chapter. "Love is patient, love is kind. It does not envy, it does not boast, it is not proud. ..."

The Love of God as defined in the Bible by Lee Smith "Old Doctrines - New Light"

John, in his writings (specifically the Gospel and his first letter), uses a circular argument to explain the interrelation between

  i. Love for God,
 ii. Love for the brethren, and,
iii. Obedience to God 

A church with a strong ‘family’ base (surprise! surprise!) will vote for ‘love for the brethren’

a more insular and individualistic fellowship will opt for ‘love for God’.

Books:
"The Four Loves", by C. S. Lewis which explores the nature of love from a Christian perspective through thought-experiments and examples from literature.
The 5 Love Languages, by Gary Chapman, marriage counselor
Works of Love, by Søren Kierkegaard written in 1847

Links:
Love-sessions.com
The 5 Love Languages web site (A different 5 - Words of Affirmation, Quality Time, Receiving Gifts, Acts of Service, and Physical Touch.) by marriage counselor Gary Chapman
How Do I Love Thee? Experts Count 8 Ways | LiveScience
  Love Language Quiz
Does the L-Word Belong in Business?
When to use the word love in a relationship