What are raster (bitmap) images?

From The Book, "Macromedia FLASH 5 Using Flash"
published by Macromedia, Inc. 2000

About vector and bitmap graphics

Computers display graphics in either vector or bitmap format.

Bitmap (raster) graphics

Bitmap graphics describe images using colored dots, called pixels, arranged within a grid. For example, the image of a leaf is described by the specific location and color value of each pixel in he grid, creating an image much in the same manner as in a mosaic.

When you edit a bitmap graphic, you modify pixels, rather than lines and curves. Bitmap graphics are resolution-dependent, because the data describing the image is fixed to a grid of a particular size. Editing a bitmap graphic can make the edges of the image ragged as pixels are redistributed within the grid. Displaying a bitmap graphic on an output device that has a lower resolution than the image itself also degrades the quality of its appearance.