Bitmap
Explanation
In computer graphics, a bitmap or pixmap is a type of memory
organization or image file format used to store digital images. The
term bitmap comes from the computer programming terminology,
meaning just a map of bits, a spatially mapped array of bits. Now,
along with pixmap, it commonly refers to the similar concept of a
spatially mapped array of pixels. Raster images in general may be
referred to as bitmaps or pixmaps, whether synthetic or
photographic, in files or memory. In certain contexts, the term
bitmap implies one bit per pixel, while pixmap is used for images
with multiple bits per pixel.[1][2]
Many graphical user interfaces use bitmaps in their built-in
graphics subsystems;[3] for example, the Microsoft Windows and OS/2
platforms' GDI subsystem, where the specific format used is the
Windows and OS/2 bitmap file format, usually named with the file
extension of .BMP (or .DIB for device-independent bitmap). Besides
BMP, other file formats that store literal bitmaps include
InterLeaved Bitmap (ILBM), Portable Bitmap (PBM), X Bitmap (XBM),
and Wireless Application Protocol Bitmap (WBMP). Similarly, most
other image file formats, such as JPEG, TIFF, PNG, and GIF, also
store bitmap images (as opposed to vector graphics), but they are
not usually referred to as bitmaps, since they use compressed
formats internally.
source: http://en.wikipedia.org/wiki/Bitmap