An interview with Phil Schiller, Apple's senior vice-president for worldwide product marketing, in Business Week, contains the following lines:
Q: Is there any digital-rights management (DRM) built into the downloads themselves, or is it all in iTunes 4? A: No, it's all built into the [iTunes] system. Our goal is to make it transparent for the user so that they never have to think about DRM. We're using a DRM technology under the hood called FairPlay. It's a DRM technology used by iTunes and QuickTime at the system level.
Which is obviously false(ish): if I try and use iTunes to open a protected AAC file downloaded from someone's shared library, it asks for authentication. The file info also shows a "purchased by" and "Apple ID" tag, which is stored in the file. As proved by the next answer:
Q: So if I burn a disk off iTunes 4, will there be some kind of identification marker so that if I place it on KaZaA it will be traceable to me? A: If you're talking about burning a regular music CD, no.But if you burn a data CD, then yes. So, can you play a protected AAC just by altering the Apple ID in the file so that it matches an ID for which your computer is authorised? Answer (a few hours later): No.