A printhead is the device within the printer that actually sprays the droplets of ink onto the paper. If it weren't for the printheads, you would always end up with a blank piece of paper! A lot of printers these days use cartridges that already have the printheads attached to them. To see a printhead, take your ink cartridge out, flip it over and look for a metal bar. That area is the printhead.
On the other hand, some older printers will have separate print heads within the printer rather than the cartridge. If this is the case, they are located right next to where the ink cartridges snap into place. If you're still not sure where your print heads are, you should be able to find some more information about them in your printer manual.