What is raster printing?

Raster print data is the uncompressed print data in 1 and 0. This data can be compressed to form jpeg image or can be re-aranged with some header info making a bitmap or tiff image. All these image formats have a header which contains information about the image, like the size of the image, the color planes used ect... The actual image data follows this image and this is the raster data. The header stores a pointer to the raster data.
When we send data to the printer, we convert this raster image given to us by the application to a printer understood language like PCL, PS, XPS, or PDF. There are different filters which can take this raster image and convert it to a particular format. Like the cups filters, rastertops, rastertotiff.