(Thanks to Lisa B from MA for the topic)
"Coated abrasives” as sandpaper is known in the industry, is a flexible backing, coated with a layer of cutting material. It is generally used for smoothing, cleaning or roughing a surface.
Today the list of materials and applications is impressively long, with most coated abrasives being used in industrial production. (Check out this list of various sanding tools specifically for woodworkers.)
In the beginning it was much simpler.
The earliest mention of “sandpaper” is from 13th Century China. The abrasive was made from crushed shells, seeds or sand, which were then bonded to parchment using natural gum.
Since then other materials have been used as an abrasive, ranging from shark skin to flint.
Glass was a common abrasive starting as early as the 1700’s. By 1769 glass-paper was being sold on the streets of Paris and people were warned of counterfeit glass-paper made with inferior abrasives.
In 1834, Isaac Fischer Jr of Springfield VT, patented a mechanized process for mass producing sandpaper. Nearly a century later, in 1916, 3M invented wet/dry paper which is used with water as a lubricant to wash away the slurry created by sanding.
By the 20th century "sand" paper was replacing "glass"paper in the US. A 1935 addition of Home Craftsman Magazine described it as:
[A]n abrasive material prepared by coating stout paper with glue and sifting fine sand over its surface before the glue sets.
Today these papers are made with garnet well as aluminum oxide, ceramic and silicon carbide among other abrasives. Each material has its own properties that suit it for a specific task. Technology continues to improve both materials and the process of applying them. As woodworkers we reap the benefit.
Aluminum Oxide is great for woodworking, because, as it's used, the heat and friction cause it to break down into more sharp edges. This makes it a good, long lasting, sandpaper.
Garnet is also a good choice for woodwork, although it is slower and wears out more quickly than aluminum oxide. It is strong and puts a good burnish on the wood, so use it for final sanding when you want a high polish.
Ceramic can also used by woodworkers but only for the toughest applications - sanding belts, for instance, may be made with ceramic abrasives.
In the past, the abrasive was randomly applied to the glue. Today, the abrasives can be electrostatically charged so all the abrasive particles line up in the same orientation. This creates a fast cutting, long lasting, product.
Furthermore, new technology has made it possible to create abrasives the size of smoke particles (!) used for “superfinishing.”
Superfinishing, also known as micromachining, can give a surface finish of 0.01 μm. A μm (micron) is 1×1000000 of a meter.
Back to woodworking:
If you look on the back of your sandpaper you will likely see a bunch of information. Sometimes the brand name and trademark name will be listed. Almost always you will see a number for the grit, followed by a letter.
Sandpaper used in the work shop is usually backed with paper, sometimes cloth. The weight of the backing is designated by that letter. For paper, the letters range from A to F, with A being the lightest and F the heaviest. Cloth goes from lightest to heaviest using J, X, Y , T and M.
Grit refers to the coarseness of the paper. The higher the number the finer the sandpaper. I usually have 60-800 grit on hand in the shop. I use 60-80 grit for hogging off material, 100-150 for smoothing a piece and 220-320 for final sanding to prep for finishing and for sanding between coats. I use higher grits when I’m after a real polish, but mostly I use it for sharpening tools.
Your paper might also be labeled open coat, meaning the paper has less abrasive coverage. Since the abrasive particles are further apart the paper is less likely to clog. Similarly, stereated sandpaper has a dry lubricant loaded to the abrasive to keep the paper from clogging.
Stereated and open coat sandpaper are good for use on oily woods with a lot of resin or for sanding off paint and other tasks that would generally clog the paper quickly.
If you aren't sure what to use, start with the grit you need, then experiment with different brands and materials. You can find a huge variety of sheets, rolls, disks, drums... and tools to use them, ranging from sanding blocks to top of the line orbiting sanders.