Some die-hard computer scientists insist the term artificial intelligence refers exclusively to this kind of system, but most people use it loosely to refer to a set of recently developed computer algorithms including not only expert systems, but also fuzzy logic, neural networks, and genetic algorithms. Expert systems are decision-making systems. One of the main criticisms of expert systems is that it's often impossible to judge whether or not a particular fact is true.
Fuzzy systems have fuzzy rules, too.
Fuzzy logic is, of course, a kind of expert system in which the facts and rules are, well, fuzzy. In nature, neural networks are neurons, some of which are connected to sensory organs, such as eyes, ears, noses, and so on. The pattern of neurons that are active and those that are not is the way the organism represents whatever it is experiencing. As a result of experience, neurons that are simultaneously active tend to grow connections to each other. Recently, computers have become fast enough to simulate the genetic process for very simple cases.