Although there's still some argument about whether computers should have anything to do with Art (with the capital "A"), there's a lot less of it now than there ever has been. However, the argument has died down a lot with the avalanche of sheer _content_ from Hollywood and Uncle Joe's basement.
This wasn't always the case. In the beginning, serious people asked serious questions like "can computers appreciate beauty?" The emerging answer seems to be that they makes a great paintbrush -- Fractal Art notwithstanding. A paintbrush makes no commentary about the quality of the work, but in the hands of a master can demonstrate the subtlest shadings and evoke the strongest emotions. The computer is a tool, and a liberating one. Your standard paintbrush also doesn't have an undo.
The first people to use a computer to generate images generally weren't interested in artistic effect. They just wanted something easier to interpret than a printout full of numbers or stacks of punch cards. In fact the, the first radiosity rendering was not actually rendered by computer. A computer calculated a set of grey values for the surfaces of objects in a scene and a human artist cut and ironed special paper together to create the scene based on the computer's calculations. [MASSON 389]
Now, thanks to the motion picture and advertising industries, computer generated imagery (CGI) is usually used to augment reality (as in adding dinosaurs) or to extol the advantages of a particular product. Any artistic merit in these happens as a comprimise between the pragmatic needs of the production, as enforced by the producers, and the artistic visions of the animators and production crew, as visualized by the art directors.
However, thanks to an influx of cheap tools for cheap personal computers art may have its chance again. Compared to the price of oils and easel and canvases, a PC and the occasional trip to the copy shop looks almost thrifty. And just like the paintbrush, the computer won't make snarky remarks about the quality of your work.
So, discarding your paintbrush (undo button still infuriatingly absent), you want to perpetrate Art (note initial capital) with the help of a computer. What's it all about then?
Right now, there are two main paradigms in computer generated imagery. The simplest is 2D and directly maps to how you might draw an image on a piece of paper with a pencil. In this case, however, the image is on the computer screen and the instrument you draw with might be a tablet stylus or a mouse, but the marks it makes will seem to be from a pencil or pen or paintbrush. The second kind is 3D, where the screen becomes a window into a virtual environment, where you arrange objects to be "photographed" by the computer. Of course the image generated is 2D, so you can always take it into your paint program for additions, much in the same way the Weekly World Inquirer Magazine inserted the space aliens in the coffee bar. Typically 2D computer graphics use rastierized images as their primary means of source data respresentations, whereas 3D computer graphics use a vectorized geometry represenatation.
If you look closely at your computer monitor, you'll see that the pictures and words are made up out of tiny dots. Each of these dots is under the control of your computer's display circuitry (except for the dots on the screen from opening your cola too close to the monitor). To generate the pictures and windows and boxes and words you see, it shuffles the data that describes these dots round and round as needed. The data behind the dots are called "pixels".
If we add a little more complexity, we can paint on it too. In particular, if we store a red, green and blue value for each pixel, we are operating in the RGB color space. Add some transparency, call it alpha, and we've got the standard four-channel model for storing 2D imagery.
[But is it art?]
MASSON - CG101, Terrence Masson, 1999, New Riders Publishing.