23. A computer program for integrating a plurality of different data sources, the computer program comprising computer executable instructions stored in a computer readable medium that when executed cause the computer to: obtain semantic information from each of the plurality of data sources; create a conceptual model for each of the plurality of data sources using said semantic information; access a secondary knowledge source having information relating the data sources to one another; and create an integrated semantic model using said conceptual models and said secondary knowledge source; wherein said semantic information comprises characterization of at least one of constraints that hold for subsets of data in the plurality of data sources and relationships that hold between the data; wherein said semantic information further comprises information expressing properties of the data that have not been explicitly encoded in an alphanumeric representation of the data or in a syntactic structure that holds together different data elements.
32. A computer program as in claim 23 wherein each of the plurality of data sources has at least a class, and wherein said integrated semantic model comprises logic statements that express an instance, a subclass, and a method relation between at least one class from a first of the plurality of databases and one or more classes from others of the plurality of databases.