Juxtaposition is a placement of two words, descriptions, or concepts literally or figuratively next to each other. The effect on the reader is like sweet and sour sauce, yen and yang: he/she senses the contrast between the two, yet notices that they, for some reason, have a fit. Sometimes the juxtaposition can be temporal, as when a character recalls the darkness of the coal mine while seeing the flash of a bomb. This is a fertile literary device. By way of some humor, juxtaposition is also the father of the oxymoron.
Here's an example for a conceit (an extended metaphor) from the poem, "The Flea" by John Donne in which a flea that bites both the speaker and his lover becomes a conceit arguing that his lover has no reason to deny him sexually, although they are not married:
Oh stay! three lives in one flea spare
Where we almost, yea more than married are.
This flea is you and I, and this
Our marriage-bed and marriage-temple is.
CONCEIT (also called a metaphysical conceit): An elaborate or unusual comparison--especially one using unlikely metaphors, simile, hyperbole, and contradiction. Before the beginning of the seventeenth century, the term conceit was a synonym for "thought" and roughly equivalent to "idea" or "concept." It gradually came to denote a fanciful idea or a particularly clever remark. In literary terms, the word denotes a fairly elaborate figure of speech, especially an extended comparison involving unlikely metaphors, similes, imagery, hyperbole, and oxymora. One of the most famous conceits is John Donne's "A Valediction: Forbidding Mourning," a poem in which Donne compares two souls in love to the points on a geometer's compass. Shakespeare also uses conceits regularly in his poetry. In Richard II, Shakespeare compares two kings competing for power to two buckets in a well, for instance. A conceit is usually classified as a subtype of metaphor. Contrast with epic simile and dyfalu.