Bringing two different things together with an explicit word, such as "like," "just as" or "seems." Unlike the metaphor, the comparison makes it clear that they are two things.
In Romeo and Juliet
Love's heralds should be thoughts, / Which ten times faster glide than the sun's beams
Thoughts are placed beside the "sun's beams" to measure the speed Juliet desires: "ten times faster than" light. The comparison sets the measure of her impatience, for whom any messenger of flesh and blood seems slow.