In this paper we investigate the application of Galois (or concept) lattices on different data sources (e.g. web documents or bibliographical items) in order to organise knowledge that can be extracted from the data. This knowledge organisation can then be used for a number of purposes (e.g. knowledge management in an organisation, document retrieval on the Web, etc.). Galois lattices can be considered as classification tools for knowledge units in concept hierarchies that can be used within a knowledge-based system. Moreover, Galois lattices can be used in parallel with domain ontologies for building more precise and more concise concept ontologies, and for guiding the knowledge discovery process.