Basically using the OS's rendering to make a raster that is then sent to the printer. The main thing the printer's driver does in this case is know how to take the bitmap and tell the printer to print the bitmap (i.e. chunking data and/or sliding the right commands into the bitmap stream)

Contrast to, say, PostScript which allow for more compact and better scaling definition of what to print. This obviously works better for quality, however for a long time the issue was you then had to have sufficient processing power on the printer itself to handle it.

[0] - Search for 'GDI Printer' for a little more info.