It's slightly more complicated than that, and you'd probably have to work in advertising to understand why.

Ad zones, campaign weighting, frequency caps, companion positioning, exclusive campaigns, delivery limitations (language, user-agent, geographic targeting, time of day, section), probability calculation, detailed statistics, and the ability to give a client a login to view their own stats. These are just a few of the features.

If all you need is a glorified image/snippet server and counter, you could probably just write one.