Raster refers to images saved in pixels as opposed to vector. A raster could also be refereed to as a bitmap image. Raster/bitmap images are generally rectangular grid of pixels, viewable on any display medium.
Common Raster Filetypes
The XBM (X Window System) is a format for saving black and white images, and the XPM (pixelmap) is used for saving color images. There are numerous other bitmap file formats out there, however they aren't very common. Very common bitmap file formats are the GIF, PNG, TIFF, and JPEG formats. TIFF is generally uncompressed, and JPEG usually has a lossy compression. PNG uses deflate lossless compression, a variant of the Lempel-Ziv compression type, GIF uses a similar Lempel-Ziv compression variant.