Justification is being seen as acceptable by God. This normally occurs at Baptism. This is an immediate process. We can lose our justification through mortal sin and need to be restored to grace. We can not earn justification and we can not work toward it, it is a gift of God.
Sanctification is a process that begins at justification and is a life-long journey. This is growing more Christlike as we live in his commandments and act out his grace in our lives.