A raster graphics image consists of a generally rectangular array of pixels, or points of color, on a computer monitor, paper, or other display device. Each pixel has a corresponding red, green, and blue value that combine to determine the colour displayed by that pixel. In this sense, typical raster graphics are said to operate in the RGB color space. This is both the raw format that computer graphics hardware uses to project an image on your monitor, and the basis for many graphics file formats.
The quality of a raster image is determined by the total number of pixels (called its resolution), and the amount of information in each pixel (often called colour depth). For example, an image that stores 24 bits of colour information per pixel (the standard for most high-quality displays in 2001) can represent smoother degrees of shading than one that only stores 15 bits per pixel, but not as smooth as one that stores 48 bits. Likewise, an image sampled at 640 x 480 pixels (therefore containing 307,200 pixels) will look rough and blocky compared to one sampled at 1280 x 1024 (1,310,720 pixels). Because it takes a large amount of data to store a high-quality image, data compression techniques are often used to reduce this size for images stored on disk. Some of these techniques actually lose information, and therefore image quality, in order to achieve a smaller file size. Compression techniques that lose information are referred to as "lossy" compression.
Raster graphics cannot be scaled (resized) up without loss of apparent quality (or more accurately, once an image is rasterized, its quality is fixed and cannot improve even on better display devices). This is in contrast to vector graphics, which easily scale to the quality of the device on which they are rendered. Raster graphics work much better than vector graphics, though, for photographs and photo-realistic images. Late 20th century computer monitors typically display about 72 to 96 pixels per inch, while modern printers can resolve 600 dots per inch or more, so working with images destined for print can be difficult or require large monitors and powerful computers. Monitors with resolutions of 200 dpi are available in late 2001 and higher resolutions are to be expected in future.
To illustrate the matter- here's the letter "J":
Look closely at it... Take a magnifying glass to it if you like -- you won't get fried, although you may see some chromatic aberration at the edges of the magnifier. For a definition of chromatic aberration, check out the Physics entries of this or other encyclopedias. You see a "J", the computer sees something more like this:
Where you see a zero, the computer instructs its video hardware to paint the current background colour. A one calls for the current foreground colour. Yes, it is actually a bit more complicated, but it all basically boils down to one bit or the other making a distinction between the colours of adjacent pixels, which together form an image. This is the basic principle behind drawing on a computer.
In 3D computer graphics, the concept of a flat raster of pixels is sometimes extended to a three dimensional volume of voxels. In this case, there is a regular grid in three dimensional space with a sample containing color information at each point in the grid. Although voxels are powerful abrstractions for dealing with complex 3D shapes, they do have large memory requirements for storing a sizable array. Consequently, vector graphics are used more frequently than voxels for producing 3D imagery.