Watercolor Pads & Blocks
Watercolor paper is sized. Sizing is a starchy glue that helps a watercolor pigment to stay on the surface of a sheet.
As a general rule, watercolor papers have more sizing than drawing or sketching or printmaking papers, so the watercolorist can work their colors longer on the surface of the paper without absorbing completely inward.
Watercolor blocks are affixed on all edges so that the paper will stay flat - no need to tape down the sheet. Watercolor pads are loose sheets of sized papers.