HP inkjet technology is really pretty amazing.  If you take a close look at how HP (also known as Hewlett Packard) inkjet cartridges work, you will see how innovative this company really is.  Ink droplets are jetted out of tiny holes in the bottom of the cartridge (called print-heads), at a speed of around 30 mph.  And the frequency that they are jetted? According to HP, more than 25,000 times per second! These print heads get electrical signals from the printer about how much ink droplets to jet, and at what specific part of the page.

The print heads are usually on the cartridge itself for most of the HP inkjet cartridges, but on some HP inkjet printers, the printheads are actually in the HP printer itself, separate from the printer ink cartridges.  HP replacement cartridges are available from vendors such as Atlantic Inkjet who sells original HP ink, and also remanufactured HP ink, and also sells HP ink refills (used by refillers who refill their cartridges).