Nozzles, or "jets", are located on the print head of your printer or on a part of your cartridge. There may be anywhere from 48 to 320 nozzles on any given print head. Each is smaller than the width of a human hair.
In an ink cartridge of a thermal inkjet printer, each nozzle is connected electronically to a heater (resistor) that heats and cools the ink inside the cartridge. When the ink is heated, a bubble is formed. When the heat is removed, the bubble bursts, sending dots of ink through the tiny nozzles and onto the paper in the printer.
In an ink cartridge of a piezo electric inkjet printer, applied voltage causes piezoelectric elements in the cartridge's print head to contract. The mechanical pressure ejects droplets of ink into the paper.
This all happens very quickly to create the dots that form print characters on your page at a rate of up to 6,000 drops per second. You should never touch the nozzle or print head assembly with your finger as this can easily clog them.