Metaphor:
A figure of speech using an implied comparison of seemingly unlike things or the substitution of one for the other, suggesting some similarity.

Example:

Shall I compare thee to a summer's day?
by
William Shakespeare

Shall I compare thee to a summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date.

Explanation:

This entire poem can be seen as a metaphor. Shakespeare compare a women to a summers day. The purpose of this is because it is easy for the reader to grasp and relate to. This metaphor is sophisticated and unique to describe someone. This unique style is what Shakespeare is known for. Another purpose of this is the metaphor helps the reader "paint the picture more clearly" and is visually more comprehensible and easy to see. Which is what Shakespeare want you to be able to do. Also clearly characterizes the women.