A conceit is a kind of metaphor that compares two very unlike things in a surprising and clever way. Often, conceits are extended metaphors that dominate an entire passage or poem.
Metaphysical poet John Donne was known for his conceits (often called metaphysical conceits). His 1635 poem "The Flea" uses a pesky little bug to talk about, um, wanting to get it on with the speaker's lady love. Sex and insects? An unlikely pair, sure, but the unexpected originality of the contrast fuels the poem's seductive humor. Plus it helps the speaker make the argument: the fact that a flea has already bitten both the speaker and the object of his affections means that she has no reason to be, um, less than welcoming in the bedroom.