Classification or decision trees are a method to deliver decisions by making choices progressing through a set of predefined paths in the form of a tree . Expert systems can be constructed from extracting rules from experts. They can also be constructed statistically by taking a number of examples and using them to build a system that makes decisions as consistent as possible with the examples provided.
As part of our AI development system, meant to be used by internal analysts to construct expertise-based systems, we included a system that used data and made rules. In general, a classification system is one that produces a discrete value and a regression tree produces continuous values. This is a simple form of machine learning. The Wikipedia has a nice overview. Here is a good survey of software that does this.