To me, strictly speaking, "slang" means nonstandard English words that are INTENTIONALLY used IN ORDER TO BE different from standard English. They begin as a kind of secret language within a group. Knowing and using them is like a secret password. It proves you belong. Outsiders aren't suppose to know them. However, slang is constantly diffusing. If rock musicians know them, soon all of their fans know them. Soon, everyone knows them. However, they still seem new, fresh, shocking, and rebellious. At this stage, people still call them "slang." Often slang words die out quickly. Once everybody knows them, they are no longer fashionable. Occasionally, slang words become permanent parts of the standard vocabulary.

Good question. Slang is informal language, often idiomatic, playful, even vulgar, that is not considered part of mainstream language. If it catches on and becomes popular with many people, it may become recognized as mainstream.

P.S. In English, the tradition is that dictionaries simply record the language as it exists. Therefore, English dictionaries will contain slang words that "everybody" knows, but they will identify them as slang. In some cases you can find the meaning of a slang word simply by looking it up in a dictionary. For example, the American Heritage dictionary includes such entries Pot, n. Slang: Marijuana.