What is raster?
raster meaning an abstraction of the real world where spatial data is expressed as a matrix of cells or pixels, with spatial position implicit in the ordering of the pixels. With the raster data model, spatial data is not continuous but divided into discrete units. This makes raster data particularly suitable for certain types of spatial operation. The term may also refer to the region of a CRT (cathode-ray tube) or LCD (liquid crystal display) monitor that is capable of rendering images
reference: Coral Reef Information System – GlossaryWhat is