Irony is any situation in which there is a contrast between what seems to be and what actually is. In verbal irony, there is incongruity between the literal meaning — the actual words that are spoken, and the implied or figurative meaning. Verbal irony may have a wide variety of tones, from light-hearted to bitter, and may be used both in everyday speech and as a rhetorical device in literature.
Sarcasm, which literally means "flesh-tearing" in Greek, is a form of verbal irony, which has harsh overtones, as its etymology implies. A sarcastic comment is one that says the opposite of what is meant with the intent of harming or insulting. For example, a person might sarcastically remark, "Well, aren't you a genius?" to a student who has failed a test.
Sarcasm should be distinguished from facetiousness, which is similar but has humorous rather than insulting intentions. The words "sarcastic" and "facetious" are often used interchangeably in casual conversation, but the two are actually very different in tone and in purpose. The distinction between the two, however, is often a matter of tone of voice rather than the actual words. If someone says, "Oh, good job," to a waiter who dropped a tray, the intention might be either harsh or innocuous — sarcastic or facetious — depending on the speaker's inflection.
Overstatement and understatement may also be forms of verbal irony, especially if done intentionally. An example of intentional overstatement, sometimes known as hyperbole, might be, "There were mountains of books there!" where "mountains" is obviously an exaggeration. Understatement is a more subtle form of verbal irony and might be delivered with a chuckle. For instance, a speaker might comment on the same large amount of books by saying, "Well, it seems you have a few books here." In both instances, what is said is not directly opposite of the actual meaning as it is in sarcasm, but it still does not line up exactly.
Middle English literature makes frequent use of understatement as a rhetorical device. In the anonymous epic poem Beowulf, the speaker says that "no small company" gathered when Beowulf said he would fight the monster Grendel. This particular type of understatement is known as litotes, meaning something is especially true by saying that its opposite is not true.