A metaphor is a figure of speech that is used to make a comparison between two things that aren't alike but do have something in common. Unlike a simile, where two things are compared directly using like or as (example of a simile is, "Her cheeks are red like a rose"), a metaphor's comparison is more indirect, usually made by stating something is something else. A metaphor is very expressive; it is not meant to be taken literally.

For example, a river and tears aren't very alike. One is a body of water in nature, while the other can be produced by our eyes. They do have one thing in common, though: both are a type of water that flows. A metaphor uses this similarity to help the writer make a point such as, "Her tears were a river flowing down her cheeks".

Metaphors help writers make a point in a more interesting way and help the reader see something from a new perspective. By describing tears as a river, for example, the writer found a creative way to describe how great the girl's sadness was and helped the reader see a similarity between tears and a river that they might not have noticed before. This makes reading more fun and interesting.

An oxymoron is also a figure of speech in which two opposite ideas are joined to create an effect. The common oxymoron phrase is a combination of an adjective proceeded by a noun with contrasting meanings, such as “seriously funny,” or “paid volunteer”.

More Info: en.wikipedia.org