What is the difference between believing in something and having faith in something?
Believing is trusting in that which is seen and faith is trusting in that which is unseen.
According to the apostle Paul:
(Heb 11:1) Now faith is the substance of things hoped for, the evidence of things not seen.
(Heb 11:2) For by it the elders obtained a good report.
(Heb 11:3) Through faith we understand that the worlds were framed by the word of God, so that things which are seen were not made of things which do appear.
Therefore faith receives that which is not evident to the natural senses as fact, while one's belief is based on that which is experienced by the natural senses.
You see believers are such because they have experienced something in the natural realm which caused them to believe.
Faith on the other hand is something that shows no evidence of its existence in the natural realm (there is no physical proof) yet becomes manifest in the natural because of faith in something that is not natural.