QUESTION: What is rasterization?
ANSWER: Rasterization is the process of converting code that describes text and graphics into the format that is required by a printer's "print engine," which is the machinery that actually puts marks on a page.
Rasterization is performed by a "raster image processor," also known as a RIP.
With some systems, the RIP is a computer that is inside the printer itself. If you print your documents on a desktop printer, your RIP is probably inside the printer.
With other systems, the RIP is separate from the printer. For example, if your company uses a large Xerox printer, the RIP is probably a software program that runs on a Unix computer or a Windows computer that is separate from but connected to the printer.
The code that gets converted is known as "page-description-language code."
See also What is a RIP?