Did you know that there are 2 different kinds of printheads? One uses the piezoelectric method of ejecting ink, the other uses thermal (or heat).
Piezoelectric printheads are usually found in Epson printers. The ink is fed into each individual nozzle by suction. There are hundreds of small nozzles in the printhead, and the printer software sends a message to each nozzle with information on on when to fire and how much ink to eject. The piezoelectric printhead relies on a squeezing action to eject a droplet of ink onto the page – basically a physical method of forming an ink droplet. When the ink is squeezed out onto the page this forms a vauum in the printhead nozzle, bringing in more ink to repeat the process.
Thermal printheads are typically found in Canon, HP, Lexmark, Olivetti and most other brands of printer. The nozzle containing ink is superheated for a minute fraction of a second, causing the ink to boil which makes an air bubble which forces the ink droplet out of the nozzle. This is why these printheads are more susceptible to burn out if allowed to run out of ink.
Each ink droplet is extremely small and thousands are fired from a printhead per second. It is a miracle of technology that a printer can work so quickly, and why the printhead is the most important (and most fragile) part of your inkjet printer.