Hyperbole is a literary technique in which a certain piece of information, feeling, or other statement is exaggerated intentionally for a certain effect. In most cases, the literal interpretation of a hyperbole could not actually be true, but the exaggeration serves to emphasize a certain point. The statement "I have a million things that I have to do today," for example, is a hyperbole — it means that the speaker has many things to do, but it is unlikely that anyone actually needs to do a million tasks in one day. Hyperbole can also be used in literature sarcastically or for the sake of humor, though it is most commonly used for emphasis.
In prose, hyperbole is generally used for the purpose of emphasis or for humor. A writer who wants to make a particular point may make that point by overstating or exaggerating it. Hyperbole can be used in descriptions to emphasize some particularly prominent feature of a character, for instance. It can also be used to describe an action that is remarkable in some way. In these and other similar cases, hyperbole is used to place emphasis on a particular action, feeling, or feature and is not meant to be taken literally.
Often, hyperbole in literature relies on imagery that can be quite humorous. While the main focus of a given use of hyperbole may simply be emphasis through overstatement, a humorous image is, intentionally or otherwise, often a secondary result. A man may be described as having "fingers like Italian sausages," for instance. While the purpose of this phrasing may be to comment on the size of the man's fingers, it relies on the humorous image of a man with thick, stubby, sausage-like fingers. Writers who use hyperbole, therefore, must be mindful of the images they rely upon, particularly if they do not want to infuse their work with humor.
Poets also commonly make use of hyperbole. It is, as in prose, generally used for emphasis, but is much more likely to be used exclusively for humor or at least to make a certain point through the use of humor. Hyperbole can also be used to emphasize a contrast: if one idea is exaggerated while another is stated normally or even understated, the result is an emphasis of the contrast between the two. This is particularly common in poetry that seeks to explore two or more opposed ideas.