On all handsaws, regardless of origin, the teeth are made either to cut with the grain (known as ripping) or across it. The rip teeth on both Western and Japanese saws are similar: fat triangles with chisel-edge tips.
Japanese crosscut teeth, on the other hand, are long and narrow, like miniature swords. Each one slices through wood fibers with three knife-sharp edges; a squat Western tooth has only two. You can easily sharpen a Western saw, but taking a file to Japanese crosscut teeth requires steady hands, deep reserves of patience, and a master temple-builder as a teacher. If you’re lacking any of these, just replace your blade when it gets dull.