A simile is a type of figurative language used to make a comparison. A simile is “a figure of speech in which two things, essentially different but thought to be alike in one or more respects,” are compared, usually using the words “like” or “as” to make the comparison (eNotes's Guide to Literary Terms). It is an indirect comparison. You are saying that something is like something, not that it is something, as in a metaphor.
From the moment Romeo first sees Juliet, he is very attracted to her. He is captivated by her beauty. One of the ways he describes her beauty is with a simile. When Romeo first sees Juliet at the ball, he compares her to a beautiful jewel.
O, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiope's ear;
Beauty too rich for use, for earth too dear! (Act I, Scene 5)
An Ehtiope is someone from Ethiopia, who therefore has dark skin. Romeo contrasts the jewel's brightness with the skin's darkness, with night being the skin and Juliet being the jewel. In other words, Romeo thinks Juliet is very eye-catching.
Romeo and Juliet meet, and they share a conversation that is basically one long metaphor. Later, Romeo sneaks into the Capulet orchard and again praises Juliet's beauty with plenty of metaphors and other types of figurative language.