In the poem "Harlem," Langston Hughes uses five similes and one metaphor to describe what can happen to a dream when it is deferred. To "defer" means to put off or delay, and so all of the comparisons describe what happens to something that is left too long.
First, the speaker asks, in a simile comparing a dried-up raisin to the "dream deferred" referenced in the first line, "Does it dry up / like a raisin the sun?" (2-3) One might imagine the original dream as a firm, plump raisin, sweet and flavorful; however, the dream deferred is more like a hard, small raisin that has dried out and become undesirable. The contrast between the two images shows us the harm in delaying a dream.
Next, the speaker asks, in a simile comparing a festering wound to the dream deferred, "Or fester like a sore-- / and then run?" (4-5) Here, the dream that is deferred is compared to something that, if left to sit without being taken care of, becomes infected and painful. The implication is that a dream deferred becomes something bad, something that hurts.
Third, the speaker asks, in a simile comparing the dream deferred to rancid meat, "Does it stink like rotten meat?" (6) Now the delayed dream is making its presence known. Its "stink" affects everyone around, and again, the comparison has shown how something good can go bad when it is not used right away.
Fourth, the speaker asks, in a simile comparing the dream deferred to an old piece of candy or pastry, "Or does it crust and sugar over-- / like a syrupy sweet?" In this case, again, something that was once perhaps positive becomes inedible and kind of disgusting ("crust[ing] over" and "syrupy" have negative connotations). A dream deferred is like this: it begins as good but then goes bad.
Fifth, the speaker suggests, in a simile comparing the dream deferred to weighty baggage, "Maybe it just sags / like a heavy load." "Sag[ging]" clearly has a negative connotation, as does "heavy load." This comparison suggests that a dream deferred begins to weigh the dreamer down, to tire them out as carrying a heavy load would do.
Finally, the speaker uses a metaphor to compare the dream deferred to a bomb: "Or does it explode?" It is notable that Hughes uses only one metaphor in the poem because metaphors are more forceful than similes; they say that something is something else rather than that something is like something else. Perhaps he uses the metaphor here because he believes this one to be more truthful than the similes, or perhaps it is the inevitable and eventual outcome of any dream deferred, regardless of what other stages it might pass through.