Scanners typically offer resolutions of 2,400dpi, 4,800dpi and 9,600dpi. Its important to realise that scanners simply aren’t capable of picking up this level of detail. The actual optical resolution of the CCDs in most modern scanners is 600 x 1,200dpi at best and all higher figures are based on interpolation.
Note that the specification of a non-uniform resolution – for example, 600 x 1200dpi – necessarily implies hardware interpolation, since the acquisition of data at 600dpi in one axis and 1200dpi in the other clearly cannot result in a square of data. At 600 x 600dpi such a scanner will interpolate the 1200dpi dimension down to 600dpi (usually done by merely running the stepper motor that moves the light bar at twice its minimum rate), or at 1200 x 1200dpi they interpolate up the x dimension. Basically, an integrated circuit chip in the scanner generates new data by taking the dots the scanner actually sees, and calculating where the dots in-between would most likely fall, using an algorithm to guess the colour of the new dots by averaging the colour of adjacent dots.
Software interpolation can increase the resolution even more than hardware interpolation. It is performed by the PC’s processor under the control of the scanner’s TWAIN driver software. The problem is that best guesses can never be truly accurate. Interpolated images will always seem too smooth and slightly out of focus. This doesn’t matter so much with line-art where interpolation has the effect of smoothing out jagged edges. But for continuous-tone images like photographs its often better to stick with a scanner’s actual optical resolution.