At the moment (2012) the most common type of printer is the inkjet printer. There are different types, one of which works in a similar way to a cathode ray oscilloscope. In this simple type, tiny droplets of ink out are fired out of a nozzle. The droplets pass between a set of charging electrodes and acquire a positive charge, then pass between a set of oppositely charged plates. A computer controls the voltage on each plate. The droplets are attracted towards the negatively charged plate and repelled from the positively charged plate. The extent of the deflection is determined by the voltage between the plates. The voltage is controlled so that the ink traces out letters and pictures as the paper moves past.
Surplus ink is collected in a gutter and recycled.
Obviously, black and white pictures can be greyscaled to produced decent quality pictures, but full colour printing is also possible, using four colours of ink – cyan, magenta, yellow and black