Aristotle defined rhetoric as the art of persuasion through all available means, and, generally speaking, the definition stuck. Rhetoric is firmly rooted in the idea that messages are delivered to specific audiences, and that successful communication requires that the speaker or writer appeal to the expectations, values, and conventions of those audiences. Rhetorical preferences have changed with the centuries, although the Modern Enlightenment's emphasis on objectivity generated a preference in speaking, writing, or even now in other communication media, to present your ideas in the clearest, most concise manner possible. Your clear, concise delivery should resonate with your audience in a way that leads them to not only believe what you are saying, but also to be persuaded to your causes.
Here is a basic primer on Rhetoric.
Key Scholars 
- Classical (Greek)
- Classical (Roman)
Related Wikibooks 
Helpful Links 
A Handbook of Rhetorical devices http://www.virtualsalt.com/rhetoric.htm