The latest Catholic Answers magazine to show up at my house (we might be behind) covers this in depth. I’d suggest reading that for a better understanding than what I will write, which basically copies what the author (Mr. Horne perhaps) says. However, the gist of it is that Catholics and Protestants are referring to different concepts with the word justification.
In the “once saved, always saved” mentality common to many Protestants, justification is seen as a one time deal. When you first come to Christ, the only thing that can save or justify you is your faith. We Catholics agree. There is nothing we can do to merit our initial justification. That must be through faith and faith alone. If you are standing before the firing squad (or suffocating on a cross) and suddenly, for the first time in your life without your parents having made the decision for you, realize that Jesus is the way, the truth, and the life and truly have faith in him, you are good to go.
Now before I get jumped for saying that, Catholics also believe in further justification throughout life, or perhaps that justification is not a one time event, but a process. Our original belief and acceptance of Jesus is through faith, but our lives need to be acting in faith, or what Catholics refer to as works. We sin, we fall from grace. Some of that is through action, some through inaction. Faith alone saves us but once. We must confess our sins, an action, and we must right our wrongs (to the best of our ability), another action. Jesus himself asks us to care for others throughout the Gospels. (Love your neighbor is a prime example.) Also, faith should be understood as a verb. If your faith is nothing more than an intellectual assent, it won’t save us. Our faith must move us to action. Through perservance in the virtues (action) we work toward a holier life and more faith so that in times of trial we may persevere in faith too.
You can find huge numbers of Bible passages to support this. James is a great go-to book. A great question to ask is, “by faith alone is mentioned but once in the Bible, so why do you think Catholics have a problem with it when, that once, it is immediately proceeded by the word NOT?” (James 2:24, though James 2:14-26 is good.) Jesus, throughout the New Testament also speaks toward works (take nearly any passage of him speaking to people, since he is commanding them to do more than just believe, even in cases where their "faith has saved them, they took action to come to Jesus) and Paul is fond of telling us what behaviors we must chose in his letters. (1 Tim 5, for example, isfull of things one must do.)
However, as I am not nearly so eloquent, please read the article. It will answer all your questions.