The expression "intellectual property" (IP) is commonly used to describe a family of legal systems that address ideas, attempting to protect those ideas with something similar to property rights. In the case of copyright, it’s not the idea but rather a given expression of the idea that’s protected. For example, copyright protected VisiCalc, but not the idea of a spreadsheet.
Property rights are an interesting choice as the basis for building protections for intellectual concepts. Ideas have some properties in common with physical objects. For example, ideas can be shared or given away. Unlike physical items, however, an idea can’t be stolen. When Mozart wrote out the score of Gregorio Allegri’s Miserere from memory, the Vatican was not deprived of the piece—simply its monopoly over the piece.
Much advertising money is spent trying to equate copyright infringement with theft. In a sense, this is a valid analogy, but there’s a fundamental difference. If you steal a car, you’ve deprived the owner of the car. If you "steal" an idea, you’ve deprived the originator of something less tangible: the exclusive ability to distribute the idea.
Copyright is one of four sets of laws that fall under the umbrella of "intellectual property." The other three are patents, trademarks, and trade secrets. Since the 1960s, people have tried intermittently to use patents, rather than copyright, to protect software. Unlike copyright, patents protect an idea. Originally intended to protect machine designs, a patent, like a copyright, is a form of bargain. The creator of a machine provides the patent office with a detailed design, in exchange for exclusivity for a short period (and contingent on the payment of a fee that increases over time). The duration of a patent is typically much shorter than that of a copyright—around 20 years.
Early software patents included a complete source code listing of the algorithm. In recent years, software patents have documented the effects rather than the processes, making such information much less valuable to the public. (The idea behind the patent was that, once it expired, anyone could build the patented machine from reading the patent.)
The form of IP that most closely mirrors the Free Software way of thinking is the trademark. A trademark is a method of protecting a reputation, rather than an idea. This concept is embodied in several Free Software licenses, including the BSD, Mozilla, and Apache licenses, which grant rights to use the code but not the names of the original authors (for marketing purposes, they still require attribution). In many situations, Free Software and trademarks fulfill the same goals, providing a way of attaching a value to a reputation. Red Hat, for example, uses both. They employ Linux developers to provide credibility to their support offerings (after all, who better to support software than the person who wrote it), and use trademark law to prevent other people from using their name to gain this benefit by proxy.