What Is Faith

Religious faith is a concept that has been defined in various ways, from trust to the biblical definition found in Hebrews:

"Now faith is the substance of things hoped for, the evidence of things not seen."

The elusive nature of its definition makes it unclear as to what religious faith truly is. Can anyone shed some light on its true nature? Furthermore, according to the bible, why should it be considered better evidence than things that can be seen?