So who is threatening the digital world anyway? Hackers? Criminals? Child pornographers? Governments? The adversaries are the same as they are in the physical world: common criminals looking for financial gain, industrial spies looking for a competitive advantage, hackers looking for secret knowledge, military-intelligence agencies looking for, well, military intelligence. People haven't changed; it's just that cyberspace is a new place to ply their trades.
We can categorize adversaries in several ways: objectives, access, resources, expertise, and risk.
Adversaries have varying objectives: raw damage, financial gain, information, and so on. This is important. The objectives of an industrial spy are different from the objectives of an organized-crime syndicate, and the countermeasures that stop the former might not even faze the latter. Understanding the objectives of likely attackers is the first step toward figuring out what countermeasures are going to be effective.
Adversaries have different levels of access; for example, an insider has much more access than someone outside the organization. Adversaries also have access to different levels of resources: some are well funded; others operate on a shoestring. Some have considerable technical expertise; others have none.
Different adversaries are willing to tolerate different levels of risk. Terrorists are often happy to die for their cause. Criminals are willing to risk jail time, but probably don't want to sacrifice ...