Semantic nets are a simple way of representing the relationships between entitities and concepts.
Frames can do the things that semantics networks do, but take a more object-oriented type approach.
Frames allow procedures called daemons to be attached to their slots greatly increasing the power of this knowledge representation method. Certainty factors and truth maintenance allow rules to deal with uncertainty. However, rules are cumbersome as a way of encoding relational knowledge and knowledge about objects. People have vast background knowledge to cope with everyday situations. We don't have to be told everything explicitly because we can call on the background knowledge.