They are image files that are defined by a checkerboard pattern, similar to viewing mosaic tiles. Raster images are limited by the number of pixels and cannot be enlarged without producing noticeably jagged, 'stair-stepped' edges. They are produced by digital cameras, scanners, and can also be created by programs like Adobe PhotoShop and CorelPHOTO-PAINT (among others). Magnification of raster files can never increase their resolution.

Resolution of raster/bitmap file never improves on magnification

Vector file resolution independent, infinite quality at any size.