My dad helped me to understand what FAITH really is when I was little. He simply wrote the word "faith" down the left-hand side of a page, and then filled in these words: "Forsaking All I Trust Him."
FAITH is something you apply when there is nothing concrete or visible to stand on...it is continuing to trust in God despite circumstances- refusing to look at what everything around you seems to be saying, and fixing your eyes on the truth that God is always bigger, and He is always good.
I read somewhere recently, "trust is believing it will rain; faith is carrying the umbrella." (source unknown)
So whatever "faith crisis" you may be going through, remember not to be like Peter and look at the waves around you- keep your eyes fixed on who you know Jesus to be, and you will not sink.