Argot consists of a specialized vocabulary that is used by a small, insular group of people, often taking the form of a clique. It is particularly associated with criminals and thieves, although people in specific regions, fandoms, and classes also use it to communicate. In a very insular group, the resulting argot can be essentially incomprehensible for people outside of the group, thus creating a very clear “us and them” division. This vocabulary can also be used, of course, to talk about illegal or questionable activities without fear of discovery.
There is a difference between argot and slang or jargon. Jargon refers to a set of terms, idioms, and concepts that are shared by people with a similar interest. It is used almost like verbal shorthand in a community of well versed individuals, such as baseball fans, computer engineers, or athletes. Jargon is not necessarily meant to be alienating, although it certainly can be confusing for people who are not versed in it. Slang is more widespread than jargon or argot, and it tends to spread quickly and metamorphose as it is used.
The term "argot" has been directly lifted from the French. In French, it refers specifically to the vocabulary used by criminals and thieves. Many French novelists, including Victor Hugo, were very interested in this type of language because they felt that it made their books more believable and interesting. The original meaning of the term has widened since it was borrowed from the French, although the association with low-lifes remains.
Using argot identifies someone as a well versed member of a group. It is meant to be deliberately alienating, and can often be derisive and derogatory when it references people outside of the group. Many groups that are marginalized anyway use language which is heavily weighted with argot to highlight their differences. As a result, many people associate the vocabulary with the lower classes, criminality, and geeks, considering it a "hermetic language" that promotes isolation rather than togetherness.
In some cases, argot may make the jump into popular culture and become slang. This is especially common with thieves' vocabulary, which includes colorful terms like "the fuzz" for law enforcement and "on the lam" for avoiding the fuzz. Since popular culture often idealizes marginal groups, terms taken from argot fall in and out of fashion, depending on what group is being idolized at the moment. People may also deliberately use it to make themselves seem "in the know," as is the case with individuals like rappers.