How and when to use Spanish Slang
Check this page for a great collection of Spanish slang.
Another interesting twist to the Spanish slang is the inversion of the syllables in a word. If someone asks to you, "¿Qué sopó, mopri?," you might not know what to think! They are really saying, "¿Qué pasó, primo?" (What's going on, man?).
This brings up interesting detail in Spanish slang: the use of words like "primo" to address people. It is similar to the use of "man" or "dude" in English. The words "tipo/a" and "hombre" are used throughout the Spanish-speaking world. Here are some local words that you might hear: "gallo" means "guy" as Chilenismo; "compa" is regularly used to address friends in the Caribbean, as in Puerto Rican Slang, and Central America; and "pana" is used to a address friends in Dominican slang.
You would be surprised at the variation in Spanish slang among Spanish-speaking countries. The word, "guagua," for example, means "baby" in Chile and "bus" in Cuba! A traffic jam is called "una cola" in Peruvian slang; in Chile it is called "un taco;" in Panama it is called "tranque;" and in Puerto Rico it is called "un tap'n."
There is always more to say about Spanish slang and the interesting facts and twists that make the language so intriguing. As such, Mexican slang also has a lot of stories and myths also have an effect on Spanish slang and are also varied across countries. Every culture has a "boogie man," and although it may seem that these stories and myths are only a part of local legend, they really do become a part of the family with for example good Argentine slang, the community, and each individual's life. More importantly, they are carried on from generation to generation as a verbal legacy.
Here is the Wiki article about Spanish Slang (careful, strong language!): http://en.wikibooks.org/wiki/Spanish/Slang