A simile is the comparison between two things using the words "like" or "as" to denote the comparison. A metaphor, on the other hand, does not use the words "like" or "as" to denote the comparison.
In Shakespeare's Sonnet 18, "Shall I Compare the to a Summer's Day," there are multiple metaphors.
1. The first metaphor appears in line one. The metaphor is the comparison between the subject of the poem to a summer's day.
Shall I compare thee to a summer's day?
2. The next metaphor appears in line five. Here, the comparison is made between the sun and a hot eye.
Sometime too hot the eye of heaven shines.
3. One final metaphor is found in line eight. The seasons of the year are compared to a change of course.
By chance or nature's changing course untrimm'd.
No similes appear in Sonnet 18.
In Sonnet 18, Shakespeare compares his beloved to a summer day, saying the loved one is lovelier and "more temperate," meaning of a more steadily pleasing temperature, than even summer's balmy climate. He uses personification, a form of metaphor that likens an inanimate or non-human item to a human by comparing the sun to a person with an "eye" (an apt metaphor as the sun does give us the light that allows us to see) that can get too hot, and with a "gold complexion" that can become "dimm'd" (go behind a cloud). The beloved, in contrast, is never too hot or too dim. The narrator continues to compare his beloved to summer, saying that unlike summer, which will "fade," the loved one never will. Death too is personified, imagined as unable "to brag" at having brought the beloved to his "shade" or home, for this lover has been immortalized in Shakespeare's sonnet.