What does it mean to say the Bible is the "Word of God" and how is it justified?

How is the phrase the "Word of God" understood (as applied to The Bible) in the Christian tradition? I assume different sects of Christianity answer this differently and I'm interested in hearing all answers, specifically with regard to the doctrine of the infallibility of The Bible. I'm familiar with 2 Timothy 3:16: > "All Scripture is inspired by God and profitable for teaching, for > reproof, for correction, for training in righteousness..." Specifically, what is the definition of "Scripture" with respect to them being the "Word of God" and why are the letters of the New Testament considered scripture?