Grain patterns typically run parallel with the length of any board or piece of plywood. Always sand parallel with the length or grain of the wood. Sanding creates fine scratches. When scratches align with the grain, the wood becomes successively smoother as you continue with finer sandpapers. Sanding perpendicular across the grain -- against the grain -- creates cross-grain scratches that can damage the surface of the wood. Cross-grain scratches can be extremely difficult to remove.
Breaking the Rules
It's not always possible to sand parallel or with the grain. The situation might occur when sanding round or carved wooden parts, on intricate inlay patterns or in tight corners or spaces where normal sanding techniques do not apply. Sanding against the grain in this instance is acceptable. Use an oscillating sander if possible. Oscillating sanders rotate in a circular motion to smooth wood. If you can't use an oscillating sander -- for example on round legs, carvings or turnings -- use circular motions by hand with a medium-grit sandpaper such as 100-grit. Follow up with successively higher grits if needed.