In computer graphics, a raster graphics image or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium. Raster images are stored in image files with varying formats (see Comparison of graphics file formats).
A bitmap corresponds bit-for-bit with an image displayed on a screen, generally in the same format used for storage in the display's video memory, or maybe as a device-independent bitmap. Bitmap is technically characterized by the width and height of the image in pixels and by the number of bits per pixel (a color depth, which determines the number of colors it can represent).
The printing and prepress industries know raster graphics as contones (from "continuous tones") and refer to vector graphics as "line work".
The word "raster" has its origins in the Latin rastrum (a rake), which is derived from radere (to scrape), and recalls metaphorically the systematic sampling of a grid-pattern of individual pixel-spaces with a view to representing an overall image.
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 in memory.
PNG IS GREAT WHY DONT WE ALL USE MORE PNG?
PNG offers truecolor up to 48 bits, whereas GIF allows only 256-color palettes. Also as an alpha channel format you can also export it with transparency (perfectly!) and import the PNG into flash where it will be fabulously transparent.