Basically, it’s a way to make a comparison between two things that are unrelated to make a point. It’s saying something *is* something else: “Class is hell and the teacher is the devil” is a metaphor.

But don’t confuse it with a simile. A simile is when you say something is *like* something else. For instance, “She is like a flower” is a simile. “She is a flower that blooms in my heart” is a metaphor (and a cheesy one at that).

A figure of speech in which a word or phrase that ordinarily designates one thing is used to designate another, thus making an implicit comparison, as in “a sea of troubles” or “All the world’s a stage” (Shakespeare).

It is when you take a word or phrase which LITERALLY means one thing and use it to symbolize a similarity to a different object or idea.

A metaphor is a comparison without using like or as.

http://www.newi.ac.uk/englishresources/workunits/ks3/langmedia/all/simandmet1.html

Angelina Jolie’s lips are balloons- Signifying that her lips are big and puffy.