A semantic network is a graphic notation for representing knowledge in patterns of
interconnected nodes and arcs. It is one way for knowledge visualization and presentation. It was used firstly for for artificial intelligence and machine translation in computer science. What is common to all semantic networks is a declarative graphic representation that can be used either to represent knowledge or to support automated systems for reasoning about knowledge.
Ontology is a representation vocabulary, specialized to some domain or subject. It is a representation of a set of concepts within a domain and the relationships between those concepts. It is used to reason about the properties of that domain, and may be used to define the domain.
You can find more in references as (specially 4):
1) Sowa, John F., ed. “Principles of Semantic Networks: Explorations in the Representation
of Knowledge”, Morgan Kaufmann Publishers, San Mateo, CA, 1991.
2) Noy N.F., McGuinnes D. L. “Ontology Development 101: A Guide to Creating Your
First Ontology”. Stanford Knowledge Systems Laboratory Technical Report KSL−01−05 and
Stanford Medical Informatics Technical Report SMI−2001−0880, March 2001.
3)María Auxilio, “An Overview of Ontologies”, Center for Research in Information and Automation Technologies Technical Report, Interactive and Cooperative Technologies Lab, Universidad De Las Américas Puebla, México, March 2003.
4) ABDEL-BADEEH M. SALEM, MARCO ALFONSE "Ontology versus Semantic Networks for Medical Knowledge Representation", ICCOMP'08 Proceedings of the 12th WSEAS international conference on Computers, 2008.