(I know my answer is similar to James Khoury's, but a longer answer is needed. Even this is not near enough, but here goes anyway...)
It is accurate, but maybe a little simplistic to say that faith is "the belief that God means what He says."
Hebrews 11:1-2 Now faith is the assurance of things hoped for, the conviction of things not seen. For by it the men of old gained approval.
At the risk of sounding too mystical or weird, faith is the assurance of things we have no other assurance of. It is the proof of things for which we have no other proof.
The bulk of Hebrews 11 gives example after example of what people have done "by faith" eventually describing the people as "men of whom the world was not worthy" (v. 38).
The implication is that faith leads people to do things that make no logical sense, yet are Right. I emphasized "Right" because I mean much more than the best or correct choice among several alternatives. I'm using "Right" to mean perfect to the degree that any deviation from that would be a failure to follow God's Will.
James is even clearer in his statement that faith always leads to action (works).
James 2:14 What use is it, my brethren, if someone says he has faith but he has no works? Can that faith save him?
As you continue reading through to the end of James 2, you get to the conclusion that "faith without works is dead" (v. 26).
It is also important to note that while faith will lead us to do the things that God approves of, it is not a guarantee of anything in this life.
Hebrews 11:39 And all these, having gained approval through their faith, did not receive what was promised