Antithesis
Antithesis is a counter-proposition that denotes a direct contrast to the original proposition. Antithesis is a way to express contrast through direct opposites. Light is the antithesis of dark, heaven is the antithesis of hell, and some would even say that cats are the antithesis of dogs.
Antithesis
Anthesis is a way of presenting contrast. Black is the antithesis of white and vice-versa.
Antithesis is also a way to describe contrasting ideas or themes: genocide is the antithesis of world peace, for example.
Why it is Used
Contrast is a very important stylistic choice to fully illustrate a concept. By explaining a concept, idea or argument with its opposite, you give your audience a 360 degree understanding of your point. Using something's opposite helps to bolster the definition about what you're speaking. By giving your audience a contrast with the opposite point of view, they have a better idea of the concept; if they do not, you can clarify further. Antithesis is a great way to lead into exactly how you want to portray an idea or argument.
How to Use Antithesis
Antithesis makes for a great way to set up your argument or idea by showing your audience the opposite. From there, you can then specifically tailor your argument to fill the void left when describing its opposite. If you are having a hard time trying to decide how to use antithesis effectively, consider the main point you are trying to make with your speech. What would the opposing point be? Use that as a springboard to begin pinning down points of contrast to give your speech stylistic texture.