"Heads" are printheads, the actual nozzles through which ink flows from the ink cartridge and onto the paper, and they are replaceable on some HP printers. Customers can purchase replacement heads for their HP printers online from the HP parts store.
All inkjet printers utilize a printhead in their operation, and some utilize two: one for black ink and one for color ink. Additionally, printheads and ink cartridges can either be independent components or joined in a compound assembly. The compound assembly is often more expensive, but less complicated. HP employs an independent printhead and ink cartridge design for some of its printers, but customers should check their printer's owner's manual to determine the design of their exact model.