In this "Theology Basics" series, I want to post some material to help us brush up on some key doctrines of our Faith. Use this as a quick devotional or as some material for a small group. I want to post material on a variety of topics including the doctrines of: sanctification, demonology, the Holy Spirit's role in the believer's life, heaven/hell, etc. I'm going to start with the doctrine of justification. This will be a series of quick snippets to help you get to know this doctrine better.
Romans 3-5 is explicit about the doctrine of justification. It is here that Paul presents an explanation of the righteousness of God and the means of man's acquisition of it. A definition of justification is closely tied to the righteousness of God and must be defined with the whole of Paul's teaching in Romans 3-5 in mind. In previous chapters, Paul determined that the extent of sin is universal and the guilt of man is all-encompassing. For this reason, in describing God's provision of righteousness for the believer, we are introduced to the word which is translated "justified" in chapter three.
When Paul uses a form of the word justified (dikaiow) he has a specific meaning in mind. Throughout the book this term juxtaposes the judgment of God according to man's sinfulness and the legal declaration of righteousness upon those who have faith in Christ. Paul was arguing that God would judge the moralist who was a hypocrite, the Gentile who was condemned by natural law, and the Jew who was condemned by the breaking of the law. So the usage of dikaiow, as it is a legal term that implies that God is the judge, to refer to God's declaration is befitting albeit unexpected given the extent of man's depravity that had been demonstrated. 
It is in this sense that justification is labeled as a forensic doctrine. In other words, justification deals with a legal proceeding. Justification according to this definition, is the judgmental act of God's deliverance of an acquittal for sin to the undeserving sinner. This is why Romans 8:33-34 states that no one can bring a charge against God's elect because it is God who justified them. 
When justified, the believer is declared in a legal sense to be acquitted of all sin but this is not the full extent of justification. An acquittal, or pardon, merely determines the individual to be neutral in the sight of God, the judge. In this declaration, God simultaneously declares that there is no condemnation for the believer and also determines that the individual is righteous in the eyes of God. In other words, the individual that is declared justified is not simply acquitted and forgiven, but determined to be perfectly righteous. 
This is not to say that justification makes righteous, but rather that a justified individual is declared to be righteous and is treated as such despite the fact that he is not. It is certain that justification is closely related to the doctrine of progressive sanctification in the believer's experience, but the two can be studied as separate entities of a greater work which is salvation.
This is not to say that justification makes righteous, but rather that a justified individual is declared to be righteous and is treated as such despite the fact that he is not.
All of Scripture
The Pauline definition is in perfect harmony with the entirety of Scripture. Abram was declared righteous in Genesis 15 in the same way that the New Testament believer is. The Davidic psalms, especially 103, 32, and 51, also imply that God is the judge and He alone has the power to pardon individuals from the consequence of sin. Philippians 1:11 states that the fruit of the believer is righteousness and 2 Corinthians 5:21 states that the believer becomes the righteousness of God. While Paul's explanation of justification is the most comprehensive description of the doctrine, other passages certainly verify his teaching.
Horst Balz and Gerhard Schneider, Exegetical Dictionary of the New Testament (Grand Rapids, Michigan: Eerdmans Publishing Company, 1999), 330-334.
 Wayne Grudem, Systematic Theology (Grand Rapids, Michigan: Zondervan Publishing House, 2000), 722-732.
 Alva J. McClain, Romans: The Gospel of God's Grace (Chicago: Moody Press, 1973), 107.
 George E. Ladd, A Theology of the New Testament (Grand Rapids, Michigan: Eerdmans Publishing Company, 1974), 437.
Gordon R. Lewis and Bruce A. Demarest, Integrative Theology (Grand Rapids, Michigan: Zondervan Publishing Company, 1996), 137-142.