A metaphor is a comparison of two unlike things. Writers use metaphors to describe something unknown or difficult to understand by comparing it to something the reader already knows. For example, saying "Bob is a machine" is a clear, concise way to explain that he is a hard worker and gets the job done precisely. Although people do not know Bob, they do know how a machine works and can understand the comparison.
A simile is like a metaphor, although not quite as direct a comparison. Similes compare the known to the unknown, but must also use the words "like" or "as." For example, "He is as slippery as an eel" connotes that someone is trying to squirm out of something and paints a negative picture without having to say all of those words. "She is like dynamite" shows that someone has an explosive personality.
Personification occurs when inanimate objects are given human characteristics. This tool is used to describe a scene and bring life to a piece of writing. "The wind whispered through the trees" means that a gentle wind was creating noise in the branches. "The heavens cried" could be used to describe a heavy rain and give a sense of a sad mood as well. Personification is a poet's tool that can be used when the author wants to engage the audience more emotionally.
Hyperbole is exaggeration to make a point. "I am as hungry as a horse" is a startling statement, making it clear that the speaker is ravenous. "I've had a million job rejections" explains just how dire the writer's job search has been in a clear, succinct way. Hyperbole is used both in writing and in casual conversation to create a clear, exaggerated effect using a minimum of words. Hyperbole invokes the figurative rather than the literal meanings of words.