The Fall is the traditional term referring to the first sin of Adam and Eve which brought judgment upon both nature and mankind. Genesis 3:1-24 gives the account of the Fall of humanity. Sin and death entered the world, and humanity has been affected by it ever since. This came about because Adam and Eve disobeyed God's commandment not to eat of the "tree of knowledge of good and evil."
Before the Fall
Before sin and death entered this world, man reflected the image of God perfectly. Although each person is still created in the image of God, it continues to be veiled due to the sinfulness of humanity. Adam and Eve walked with God and enjoyed perfect fellowship with him.
After the Fall
"The effects of this first sin upon our first parents themselves were (1) "shame, a sense of degradation and pollution; (2) dread of the displeasure of God, or a sense of guilt, and the consequent desire to hide from his presence. These effects were unavoidable. They prove the loss not only of innocence but of original righteousness, and, with it, of the favour and fellowship of God. The state therefore to which Adam was reduced by his disobedience, so far as his subjective condition is concerned, was analogous to that of the fallen angels. He was entirely and absolutely ruined" (Hodge's Theology)." 
Why God allowed the Fall
"Commenting on John 9:3, Ryle well remarks: "A deep and instructive principle lies in these words. They surely throw some light on that great question, the origin of evil. God has thought fit to allow evil to exist in order that he may have a platform for showing his mercy, grace, and compassion. If man had never fallen there would have been no opportunity of showing divine mercy. But by permitting evil, mysterious as it seems, God's works of grace, mercy, and wisdom in saving sinners have been wonderfully manifested to all his creatures. The redeeming of the church of elect sinners is the means of 'showing to principalities and powers the manifold wisdom of God' (Eph. 3:10). Without the Fall we should have known nothing of the Cross and the Gospel."