"Yet he did not waver through unbelief regarding the promise of God, but was strengthened in his faith and gave glory to God, being fully persuaded that God had power to do what he had promised." - Romans 4:20-21 (NIV)
I usually only think about faith as something that justifies you before God - something that once you exercise it, you are made right with God and forgiven for your sins. This is most often how it's talked about in the church. As I read this, however, I see that Abraham's (who this verse is referring to) belief that God would provide a child to him (v. 17-19) is what Paul is talking about when he says faith. This belief that God will act is the faith that made Abraham righteous.
Faith is not just something we exercise once to get salvation but something we exercise moment by moment and get the fruits of a life lived with God. Any other kind of faith isn't faith - it's something you said or believed at one point. Faith means living as if what God says is true, every day, not just one time when you said a prayer in the past. It's not easy to live like that, with all the uncertainties of life and the whole fact that God is invisible - but it is absolutely essential to living the kind of life we were meant to live.