1. A method for retrieving information from databases, said databases being structured or unstructured, said databases being homogeneous or heterogeneous, wherein retrieval is performed through visual queries on dynamic taxonomies, said dynamic taxonomies being an organization of concepts that ranges from a most general concept to a most specific concept, said concepts and their generalization or specialization relationships being called an intension, items in said databases being classified under one or more concepts, said items and their classification being called an extension, said method comprising: using a computer for providing a taxonomy for said retrieval; using the computer for operating on a selected subset of interest of said taxonomy in order to refine said retrieval, said selected subset of interest being specified by using the computer for combining selected taxonomy concepts through boolean operations or being specified through querying methods, said querying methods retrieving classified items according to different selection criteria; providing a reduced taxonomy for said selected subset of interest, said reduced taxonomy being derived from said taxonomy by using the computer for eliminating from the extension of said taxonomy all items not in said selected subset of interest and by pruning concepts under which no item in said selected subset of interest is classified; and using the computer for iteratively repeating said steps of operating on a selected subset of interest and of providing a reduced taxonomy to further refine said retrieval, wherein: said step of pruning concepts includes eliminating from the taxonomy all the concepts under which no item in the selected subset of interest is classified, or preventing said concepts from being selected in order to specify interest sets; said step of providing a reduced taxonomy either reports only the concepts belonging to the reduced taxonomy or, for each such concept also reports how many items in the interest set are classified under the concept; said intension is organized as a hierarchy of concepts or as a directed acyclic graph of concepts, thereby allowing a concept to have multiple fathers; items in said classification are classified programmatically or automatically; in said extension, there exists at least one item such that said item is classified under at least two different concepts such that each of said two concepts is neither an ancestor nor a descendant of the other concept in the intension; zero or more concepts represent a tag cloud, said tag cloud having as descendants all or parts of the terms or phrases that is derived from the items, each tag cloud and each of descendents is used as a dynamic taxonomy concept to define a subset of interest possibly in combination with other clouds or concepts or querying methods, each tag cloud and each of descendants is used as a dynamic taxonomy concept to summarize a subset of interest; and said method to construct and provide through a reduced taxonomy the relationships between any two concepts based on the classification by using the computer, a relationship between any two concepts existing if at least one item is classified (1) under a first concept or any descendants of the first concept, and (2) under a second concept, or any descendants of the second concept.