[1] In its simplest form, it is a block of wood or cork with one smooth flat side.
The user wraps the sandpaper around the block, and holds it in place (by inserting a fitted piece of cardboard under the sandpaper, one can soften the impact on the wood and protect against tears or uneven wear on the sandpaper).
Fancier versions use clips, teeth or clamps to hold the paper in place.
They are usually sized to hold a quarter or half sheet of sandpaper.
Construction workers often use commercial one-piece sanding blocks consisting of a foam plastic block with an abrasive coating.