1. A method for comparing a first document and a second document, the method comprising: associating, by a processor, a respective weight with each of a plurality of information types including text-based information, graphical information, audio information, or video information; identifying, for each of the first document and the second document, by the processor, one or more segments each corresponding to one of the plurality of information types; and estimating, by the processor, a similarity value between the first document and the second document, by comparing each segment of the first document with a segment of the second document that corresponds to a same information type, wherein the similarity value is based on a distance, in a semantic hierarchy, between a first semantic class associated with the first document and a common ancestor, in the semantic hierarchy, of the first semantic class and a second semantic class associated with a second document; and combining results of the comparison based on the respective associated weights.