The principle of the continuous liquid ink jet process is precisely what the name implies: a steady stream of ink droplets formed at high rates is ejected from an orifice in the direction of the receptive medium. Those drops that are not meant to reach the substrate (the non-printing drops) are intercepted and recycled. The drops destined to form an image (the printing drops), on the other hand, are deflected in mid flight, thus passing by the interceptor and flying onwards to finally hit the substrate, forming an ink dot. Some systems deflect those drops that are to be recycled and allow printing drops to pass. The rate of droplets issued from the nozzle is controlled by a vibrating piezoelectric crystal that can form hundreds of thousands of individual droplets each second.

Ink-jet Printing Development Path
Ink-jet is a non-impact dot-matrix printing technology in which droplets of ink are jetted from a small aperture directly to a specified position on a media to create an image. The mechanism by which a liquid stream breaks up into droplets was described1 by Lord Rayleigh in 1878. In 1951, Elmqvist of Seimens patented the first practical Rayleigh break-up ink-jet device.2 This invention led to the introduction of the Mingograph, one of the first commercial ink-jet chart recorders for analog voltage signals. In the early 1960s, Dr. Sweet of Stanford University demonstrated that by applying a pressure wave pattern to an orifice, the ink stream could be broken into droplets of uniform size and spacing.3 When the drop break-off mechanism was controlled, an electric charge could be impressed on the drops selectively and reliably as they formed out of the continuous ink stream. The charged drops when passing through the electric field were deflected into a gutter for recirculation, and those uncharged drops could fly directly onto the media to form an image.4 This printing process is known as a continuous ink-jet.

While continuous ink-jet development was intense, the development of a drop-on-demand ink-jet method was also popularized. A drop-on-demand device ejects ink droplets only when they are used in imaging on the media. This approach eliminates the complexity of drop charging and deflection hardware as well as the inherent unreliability of the ink recirculation systems required for the continuous ink-jet technology.

http://www.imaging.org/resources/leinkjet/part1.cfm

There are two categories of inkjet printers and inkjet cartridges with which to familiarize yourself--continuous inkjet printers and drop-on-demand inkjet printers.

As the name suggests, continuous inkjet printer cartridges spray a continuous stream of ink droplets onto the paper. Continuous inkjet printers allow for drops to be generated at extremely high speeds. This type of inkjet printer and inkjet cartridge system creates photographic quality printer outputs. The downside is expense.

Unlike continuous inkjet printer cartridges that spray a continuous inkjet stream, drop-on-demand (DOD) inkjet printer cartridges only release ink droplets as required.

http://www.inkcartridge.com/inkjet.aspx