Ontology (information science) - Formal representation of knowledge as a set of concepts within a domain. May or may not be hierarchical.
Taxonomy - A scheme of classification, especially a hierarchical classification, in which things are organized into groups or types.
Note that ontology (information science) is not the same as ontology (philosophy), which deals with concepts such as existence, being, becoming, and reality.
Also see Tom Gruber's writeups of ontology (information science) from 1992/1993 and 2009:
And Kurt Cagle's compare/contrast of taxonomy and ontology:
And lastly an older paper, with nothing particularly pretty, but a good survey of many types of ontology visualization, Ontology Visualization Methods – A Survey (PDF) (2007)