Little to add to what Lukus explained so well already. Just one (not so minor) point - natural Japanese stones seem to work perfectly for traditional plain carbon steel blades. But not quite so well for alloyed steels, let alone some hard stainless steels. Synthetic stones, ceramic rods or diamonds tend to be a lot better choice for the latter unless you don't mind having to work several times as long to get a good edge. That's also why Japanese water stones don't last long if you use them for sharpening hard, heavily alloyed steels.