The differences between justification and sanctification can be summed up as follows:
Justification is all of God. Man plays no part. But whilst sanctification is also the work of God, we are expected to co-operate.
Justification is instantaneous; it is a once-for-all thing. You are as justified now as you will ever be, and you are as justified as any other Christian. Sanctification, however, is a process. It begins at salvation and goes on for the rest of our life. The Christian can grow in sanctification, and he can also decline in it (backsliding). All Christians are at different stages of sanctification.
Justification makes us acceptable to God. Sanctification is to make us like Jesus. Union with Christ is the foundation of sanctification. It is because we are in Christ that we are called to be like Christ.