from Vine's Expository Dictionary of New Testament Words (McLean, VA: MacDonald Publishing Co, no date)

AGAPAO and the corresponding noun agape present "the characteristic word of Christianity, and since the Spirit of revelation has used it to express ideas previously unknown, enquiry into its use, whether in Greek literature or in the Septuagint, throws but little light upon its distinctive meaning in the N.T. [New Testament].

"Agape and agapao are used in the N.T. (a) to describe the attitude of God toward His Son, John 17:26; the human race, generally, John 3:16; Rom 5:8; and to such as believe on the Lord Jesus Christ, particularly, John 14:21; (b) to convey His will to His children concerning their attitude one toward another, John 13:34, and toward all men, I Thess. 3:12; I Cor 16:14, 2 Pet 1:7; (c) to express the essential nature of God, 1 John 4:8.

"Love can be known only from the actions it prompts. God's love is seen in the gift of his Son, I John 4:9, 10. But obviously this is not the love of complacency, or affection, that is, it was not drawn out by any excellency in its objects, Rom. 5:8. It was an exercise of the Divine will in deliberate choice, made without assignable cause save that which lies in the nature of God Himself, cp. Deut. 7:7,8.

"Love had its perfect expression among men in the Lord Jesus Christ, 2 Cor. 5:14; Eph. 2:4; 3:19; 5:2; Christian love is the fruit of His Spirit in the Christian, Gal. 5:22.

"Christian love has God for its primary object, and expresses itself first of all in implicit obedience to His commandments, John 14:15, 21, 23; 15:10; I John 2:5; 5:3; 2 John 6. Self-will, that is self-pleasing, is the negation of love to God.

"Christian love, whether exercised toward the brethren, or toward men generally, is not an impulse from the feelings, it does not always run with the natural inclinations, nor does it spend itself only upon those for whom some affinity is discovered. Love seeks the welfare of all, Rom. 15:2, and works no ill to any, 13:8-10; love seeks opportunity to do good to 'all men, and especially toward them that are of the household of the faith,' Gal. 6:10. See further I Cor. 13 and Col 3:12-14."

In repect of agapao as used of God, it expresses the deep and constant love and interest of a perfect Being towards entirely unworthy objects, producing and fostering reverential love in them towards the Giver, and a practical love towards those who are partakers of the same, and a desire to help others to seek the Giver.

Note: Vine's wording in quotations above indicate references from "Notes on Thessalonians" by Hogg and Vine, page 105.

For another excellent description of agape love by G.B. Caird, in his commentary on Saint Luke, browse:

http://www.bibletexts.com/terms/agape-c.htm