Sandpaper is paper (imagine that) with abrasive grit adhered to one side. When rubbed across a surface, the grit removes small amounts of material, unveiling a smoother surface. Sandpaper is categorized according to the number of abrasive particles per square inch of paper. The larger the number, the more particles there are. The smaller the number, the fewer particles there are.
So how does this translate to your next project? Sandpaper grit numbers range from 24 to 1,000. For initial sanding on a project and quick removal of offensive material, choose something in the 24 to 50 range. For prepping wood for fine finishing or removing varnish, go with a grit rated either 60 or 80. Fine sanding calls for grit rated from 100 to 220, which is appropriate for working with completely bare wood. Grits rated from 220 to 1,000 are used in the varying stages of final polishing.