A *metaphor* is a figure of speech which compares something with another, generally unrelated thing by virtue of certain shared characteristics: *Shall I Compare Thee to a summer’s Day*.
A metaphor is a type of analogy, related to the simile, allegory, and hyperbole. It describes a subject by comparing certain features to those of another, generally unrelated subject. It is a literary, rather than a literal, comparison. Whereas a simile states that X is similar to Y (typically with the explicit use of the words like or as), a metaphor suggests that X and Y are equivalent at the point of comparison.
Metaphors are the basis for many idioms. For example, phone tag is a situation where one party attempts to reach another by telephone, fails, and requests a call back; the other party calls back, fails to reach the first, and requests a call back; and so on. It is a metaphor for the playgorund game of tag, in which one children chase one another in turn.