The term "Positive Thinking" originated with Norman Vincent Peale back in the 1950's. Positive Thinking is not "faith". If you had been on the Titanic when it went down, all of the positive thinking that you could muster up would not have saved you! So, what is the difference? Hebrews 11:1 says that faith is the substance of things not yet seen....faith has substance and positive thinking is only a thought. Is there anything wrong with positive words? No, we should speak them constantly to build up the believers or as a parent, to build up our children.
Confusing the two though can be the difference between a true doctrine and a false one. Kenneth Copeland believes if you speak it, those words will come to pass. He is a false teacher and this is not the doctrine of Jesus. People who use positive thinking to accomplish what faith does will always be disappointed.
Faith is simply trusting God.