Slang are words that are informal. Usually each generation or social group has its own slang - for example, older people can have trouble understanding the slang of younger people. On the other hand, younger people often understand, but find silly or old-fashioned, the slang of older people.

Over time, language tends to get more complex, since new words enter much faster than old words leave. Over time, slang almost always becomes part of the language, and approved for use by all.