Similes (comparisons using "like" or "as") and metaphors (direct comparisons without using "like" or "as") are wonderful tools by which an author relates something familiar to something unfamiliar in order to make the audience understand more clearly the unfamiliar item. For example, in John Donne's "A Valediction: Forbidding Mourning," he uses two metaphors. In one, he compares his love for his wife to a compass. When he leaves, she is the "fixed foot" of the compass which keeps him grounded, and he draws away from the center (home) to his destination. She leans toward him, and he is forever connected to her. As his trip comes full circle (as is the purpose of a compass), they are reunited and the compass is once again together and erect--the two become one again. The other is that their love is like gold. You can beat the gold into very thin strips, but it never breaks apart; so it is with their love. It is strong and will never break even though they may be apart from one another.
Other examples are: You are as stubborn as a mule.
He is as graceful as a willow.
Her eyes are black diamonds.
He is the Devil.
My sister's arrogance is second only to Julius Caesar's.
I hope this helps you to understand this wonderful tool of figurative language, namely simile and metaphor.