The Differences between Justification & Sanctification

Justification describes the position of acceptance with God which he gives us when we trust in Christ as our Saviour. It is a legal term, borrowed from the lawcourts, and its opposite is condemnation. To justify is to acquit, to declare an accused person to be just, not guilty. So the divine judge, because his Son has borne our condemnation, justifies us, pronouncing us righteous in his sight. 'Therefore, there is now no condemnation for those who are in Christ Jesus' (Rom. 8:1).
Sanctification, on the other hand, describes the process by which justified Christians are changed into the likeness of Christ. When God justifies us, he declares us righteous through Christ's death for us; when he sanctifies us, he makes us righteous through the power of his Holy Spirit within us.
Justification concerns our outward status of acceptance with God; sanctification concerns our inward growth in holiness of character.
Further, whereas our justification is sudden and complete, so that we shall never be more justified than we were on the day of our conversion, our sanctification is gradual and incomplete. It takes a few moments only in court for a judge to pronounce his verdict and for the accused to be acquitted; it takes a lifetime even to approach Christlikeness.

--John Stott