In order to understand which element best suits your project it is import to understand what they are and what they can do.
Dictionary
In a Lettria project, list of words having a common point and intended to be identified in data.
Example
A dictionary of rude words and a dictionary of medical words can be used to avoid coarseness and protect privacy on networks.
Taxonomy
A taxonomy is a hierarchical classification of things or ideas. Unlike ontologies, taxonomies do not contain properties (relationships and attributes).
Example
The classification of music could have separate facets organized by genre, year created and record label.
Ontology
Knowledge representation system, usually more complex than a taxonomy, used to organize classes of things or concepts in a hierarchical way and to assign them properties.
Example
Ontologies can be used to extract information to fill out databases and CRMs.