Thermal Inkjets: Most consumer inkjet printers, such as Canon, HP, and Lexmark (but not Epson printers) use a thermal inkjet; inside each partition of the ink reservoir is a heating element with a tiny metal plate or resistor. In response to a signal given by the printer, a tiny current flows through the metal or resistor making it warm up, and the ink immediately surrounding the heated plate is vapourised into a tiny air bubble inside the nozzle. As a consequence, the total volume of the ink exceeds that of the nozzle. An ink droplet is forced out of the cartridge nozzle onto the paper. This process takes a matter of milliseconds.
The printing depends on the smooth flow of ink, which can be hindered if the ink begins to dry at the print head, as can happen when an ink level becomes low; dried ink can be cleaned, from a cartridge print head, by gentle rubbing with isopropyl alcohol on a swab or folded paper towel.
The ink also acts as a coolant to protect the metal-plate heating elements: when the ink supply is depleted, and printing is attempted, the heating elements in thermal cartridges often burn out, permanently damaging the print head. When the ink first begins to run thin, the cartridge should be refilled or replaced, to avoid over-heating damage to the print-head; see more at: inkjet printer.
Piezoelectric Inkjets: All Epson printers use a piezoelectric crystal in each nozzle instead of a heating element. When current is applied, the crystal changes shape or size, forcing a droplet of ink from the nozzle. A piezoelectric inkjet allows a wider variety of inks in a much finer quality than thermal inkjets, while more economical in ink usage.
From enwikipedia.org