Imagery is the use of figurative language, and it is something that "paints" or illuminates an image in one's mind as he or she reads. It can be found in music, poetry, short stories, etc. The better the imagery, the easier it is for the reader to imagine the setting, the characters and the plot development.
There are several literary devices that create imagery for the reader. One of these devices is a metaphor, which compares two dissimilar things that have similar characteristics. In the first line of "Catch the Moon" by Judith Ortiz Cofer, there is a description that provides imagery using a metaphor:
Luis Cintron sits on top of a six-foot pile of hubcaps and watches his father walk away into the steel jungle of his car junkyard.
The metaphor is the "steel jungle" which compares the collection of steel objects piled up and crowding the area to leaves, trees, vines and other plants that obscure the way when entering into, or traveling through, a jungle.
Another literary device that provides imagery is found in the fourth sentence. It is called hyperbole which is...
...obvious and deliberate exaggeration or an extravagant statement. It is a figure of speech not intended to be taken literally since it is exaggeration for the sake of emphasis.
Hyperbole is seen as the narrator describes the woman whose house Luis broke into:
...the old lady with a million cats was a light sleeper...
Obviously, the woman did not have a million cats; the exaggeration lets us know that she had more than one or two cats. The narrator may also be trying to imply that she is one of "those" people that has more cats than rooms to keep them or food to feed them—the implication is often that someone with so many animals is a little crazy. This is not necessarily the case, but we might infer that this is the author's implication in making a point of mentioning the enormous number of cats the woman has.
Sometimes imagery can be conveyed using sensory details, which are details or specifics words or phrases that appeal to one of the five senses. In the following, the sensory details included appeal to the sense of sound and vision:
Luis heard a car drive up and someone honk their horn. His father emerged from inside a new red Mustang that had been totaled.
In this description the first thing that helps create imagery is the "honk" of the horn. ("Honk," itself, is imagery, using the literary device of onomatopoeia, which is a word that describes the sound it stands for.) In this case, we can almost hear the horn. The second image refers to the car: it is red and new (so it's also shiny), and it has been totaled, so we can imagine the side or fender crushed, with perhaps seams of grey metal showing.
In each of these cases, the descriptions provided created images (or imagery) in the reader's mind to make the scenes and events of the story more alive—more realistic.