It's confusing to know the origin of denims and jeans. The denim is believed to be one of the oldest fabrics that came from France, "serge de nimes." But there is also one fabric named "Nim," made partly of wool, which is believed to be also from France produced in the 17th century. While "serge de nimes" is made of silk and wool, and "nim" is made partly of wool, "denim" is a cotton fabric.
Jeans, which was originally composed of cotton, linen, and wool to make that durable fabric, came from Genoa, Italy. It was largely produced in Lancashire by 16th century. But during the 18th century, jean was now made solely of cotton and was used to make men's clothing.
The main difference between jeans and denims is that jean is made of two threads with the same color while denim is made of two threads of different colors (but one thread should always be white).
Jeans and Denims have matured through the years. Giant companies like Levi's, Rough Rider Jeans, Lee, Bobson, and the like have taken steps to improve the style and comfort of jeans and denims. To be competent enough, these companies have made sure that they are a cut (or style) above the rest.