Atanu Dey provides a tutorial. A summary:
Depending upon the size and complexity of what a software program does, you have to employ a bunch of programmers and have some computers for them to do their stuff on. That is the fixed cost of producing the software program. That fixed is high in the case of operating systems and huge database programs if you have to pay programmers fat salaries. If you got a whole bunch of people to collaborate and produce software for free, then you have low fixed costs. The latter is what mostly happened in the case of Linux while the former is what happens in a commercial shop like Microsoft.
Once you have produced the software program with whatever fixed costs — high, low, or intermediate — you then make a certain number of copies. Depending on the cost of the medium you use to distribute the software, your variable costs (and therefore the marginal costs of distribution) is determined. If you use punched cards or paper, marginal cost of distribution is high; if you use cds, it is low; it is lowest so far if you send it over electronically by wires or wirelessly. Because electronic distribution of software is so cheap over the internet, you can say that the marginal cost of production as well as distribution comes pretty close to zero.
Information goods are essentially different from material goods. That is so because of two reasons. First, the nature of information goods itself. And second, the cost of production of IGs. IGs have public goods characteristics, they have externalities, and have high fixed costs. All these are deviations from the conditions required for competitive markets. Thus markets will not deliver the social welfare maximizing outcome. All sorts of distortions will result such as the presense of monopolies. Monopolies, like everyone and his brother, maximize profits and since they have market power, they can charge whatever suits their fancy. So they can price an operating system at $300 a pop which is way above the marginal cost of production (exactly $0) and the marginal cost of distribution (nearly $0).
So is the economics of software essentially different from the economics of other goods? Not really. There are differences in their associated costs (fixed, marginal, average) and the ways in which they deviate from the conditions required for a competitive market. But the fact remains that there is nothing surprising about the way the market for software behaves if one were to understand the nature of software and the nature of markets. Like shoes and ships and sealing wax, they follow predictable pathways in the marketplace.