Paper is a very old product that has been in use since ancient times. In the earliest times, people were happy with whatever paper they could create and not much concern was put into the finish and weights, or if it was FSC, recycled or what ever. When you made paper from animal skins, jute, banana leaves and rocks you were happy with whatever you got!
Over the centuries paper making was perfected and various standard sizes and types of paper evolved for different uses. For example, books were a new thing a few hundred years ago and the common sizes that evolved were based on variations of folding down a sheet of paper that was about 25 x 38 inches. This “basic” size of paper became known as “Book” paper. Along those same lines we get the “basic weight”.
The paper that was made was sold by the “One Hundred” weight, or what we call CWT. The printers needed to tell the paper maker what they wanted so they would specify that they wanted paper that weighed a certain amount when weighed at a certain size. This evolved into giving paper names like 60# book, which meant “I want paper that is 25 x 38 inches and I want a ream of it to weigh 60 lbs”. (60# basic weight when weighed at the basic size).
In case you are wondering, a ream is 20 “quires”. A quire is usually 25 sheets. So a ream is 20 x 25 = 500 sheets.
As time went by more sizes became standardized so that today we have a bunch of “basic” sizes for the various papers we use every day. We don’t always or even often use the paper at it’s “basic” size. Just remember that the basic size is the one that goes along with the basic weight.
Here is a reference to paper sizes and weights. There is tons more information such as this available on-line. Just search for “Paper Basic Weights and Sizes”