Unlike our normal paper that we use today Chinese paper was VERY thin and translucent (partly see-through.) So they could only write on one side of paper, it was just too thin to write on both sides.
The invention of paper spread slowly outside of China to other East Asian countries and cities. Even after seeing paper, people could not figure out how to make it by themselves! Too bad for them. The people then demanded that they learn the manufacturing of paper, but China refused to give away their secret of papermaking. They were reluctant to give up their secrets of making paper.
After more commercial trading and the defeat of the Chinese in the Battle of Talas (they were defeated by the Arab Abbasids, the war was about the control over central Asia), the paper invention went all around the Middle East. Production started in Baghdad , the Arabs invented a way to make a thicker sheet of paper.
This papermaking had spread to Damascus by the time of the first crusade, the war had interrupted the paper production. It split the production into two centers:
Iran: This was the center of thinner papers which was adopted by India.
Ciaro: This was the center that kept making thicker paper.
Then the first paper mill in Europe was developed in Spain. In Spain the mill was in the city of Xavia (modern day Valencia) in the year 1120. Later more mills were built in Fabriano, Italy. This was in about the 13th century when paper was first introduced to Europe. The Europeans used linen and hemp rags as a source of fiber. The oldest paper document known in the West is the Mozarab Missal of Silos. (This was probably written in Islamic Spain.)
Paper in our life today differs much from the old style of Chinese paper, paper now is thin (but not too thin) material that is produced by the amalgamation of fibers, usually vegetable fibers. Vegetable fibers usually contain cellulose which are subsequently held all together by a process called hydrogen bonding. Sometimes the fibers used can be synthetic, but usually the fibers are natural.
The most used fiber is wood pulp, which is from (of course) pulpwood trees. But sometimes companies will use softwoods, hardwoods, spruce and aspen trees as well. Other kinds of vegetable fibers are: Linen, hemp, cotton and rice.
Papermaking is very complicated and long to describe, so here is a quick synopsis of how they make the paper...
Fiber Processing/Pulping is when certain fibers are split from each other and carbohydrate surfaces are exposed. Hydrogen bonding between these carbohydrate surfaces gives paper its strength. Fibers can be split three different ways: mechanically, chemically or a combination of the two.
Drying is what must happen after the paper-web has been produced, the water in it must be removed for it to be a regular and usable piece of paper. This is all done by forcing and pressing and then drying the paper. This takes out all of the water and gives you the paper that you use so much today.
Like I said in the beginning, Cai's invention is considered one of the most amazing and important inventions of all time, mostly because paper enabled China to create and develop their civilization much faster, unlike their old materials (bone, silk, bamboo etc...) This did the same for Europe when paper was introduced to Europe in the 12th (or the 13th) century, and then in the U.S. many years later.
(BUT! There have been studies in 2006 that show specimens bearing certain writings on them).
So we don't know if Cai-Lun was the FIRST person to use paper, but we do know he was the one who started the whole trend of using paper all around the world today. So really, our thanks does belong to Cai-Lun, because without him paper would have never been invented, and our civilization would be almost IMPOSSIBLE!