Slang is casual spoken language which differs from dialectical speech and jargon as well as formal speech. Some linguists think of it as the sprinkles of color in a language, since it is often unique, unusual, and sometimes startling. As a general rule, slang is not used in formal spoken language, or in writing, unless the speaker is attempting to achieve a deliberate effect. Some slang terms, however, make the jump to accepted common usage, as was the case with “OK.”
Often, slang begins as group-specific argot, which is related to jargon. Unlike jargon, however, argot is not a collection of technical terms that are used within a group with limited outside understanding. Instead, it is used to differentiate members of a group from others, and to foster a sense of collective belonging in the group. In addition, it can provide a way to talk about questionable or illegal activity without being obvious. When argot begins to spread from small subgroups to the rest of a group of language speakers, it becomes slang.
Typically, slang is humorous, ribald, or shocking. It is designed to make speech more casual and playful, and it may not always be suitable for polite company. Many terms are related to human sexuality, for example, and slang can get rather graphic. The terms are also usually short-lived, rarely existing even long enough to make it into the dictionary. There are, of course, exceptions to this rule; English speakers have been saying “beat it” for centuries, for example.
Generally, slang diffuses through a group of people, and it may spread out across an entire region or class. People who are not in that group may pick up the terminology as well, causing the usage to spread, or the words may remain isolated to a smaller subgroup. College students, for example, often develop complex slang terms, with words from different regions diffusing on a college campus. As these students leave, they carry these terms to other young people in various parts of the world, often making the speech of the younger generation incomprehensible to those outside of it.
Learning how to use slang properly can be very difficult. Many new language learners struggle with casual language and idioms because the terms are often nonsensical and difficult to comprehend. If someone can master this aspect of a language, however, they can fit in much more effectively with native speakers.