It's very common to hear Spanish speaking folks from South America use the word "vos", meaning "you" instead of "tu" or "usted", the most known uses for such a word. The word "vos", which is commonly used in the southern most countries such as Argentina, Chile and Uruguay date back to the Spanish conquest a few centuries back, and it's a direct derivative from the word "vosotros" which is a trademark from the Castellano (the actual name for the Spanish language); vosotros is used to refer to them, or you (plural) in either past, present or future tense.
It was recorded during the 17th century that some catholic priests that traveled to South America started using the shortened-version of the word vosotros, so that the natives could understand them better and so that they could even pronounce the word. It's not entirely clear yet how this modified version was transferred and incorporated into the other southern provinces. We can only assume that literature had something to do with it and even changes in command from the Spanish Oligarchy from one point to another facilitated this wide-spread of the term.
Not very long after that, other countries in Central America adopted such term, which interestingly enough, didn't make its way to the northern part of the continent (Mexico, etc.) or the Caribbean for that matter. In countries such as Guatemala, the adjective became popular through some publications used by elementary schools from Argentinean editorials in the early part of the twentieth century, making it adaptable to the next generation of students.
Central American countries still use the term today, but not with the same fashion as intended to be, the term is only used with peers and people highly trusted in the most immediate of circles. We won't see a teenager speaking to an elderly adult that he or her might not know using the "vos" because it would be deemed as inappropriate.
The term has been very characteristic of the southern part of the continent to such a degree, that just by the sole use of it, it is commonly assumed to identify South America.
Here's an example of the word "vos" being used and its regular version (interpretation):
Man: "Estaba sentado viendo la television pero luego llegaste vos y me hablaste, lo cual me hizo perder atencion al programa que queria ver".
Man 2: "Primo, yo no queria distraerte a vos, pero tengo algo muy importante que decirte. Vos sabes que no te interrumpiria si no fuera completamente necesario".
Normal version man: "Estaba sentado viendo la television pero luego llegaste y me hablaste, lo cual me hizo perder atencion al programa que estaba viendo".
Normal version man 2: "Primo, yo no quiero distraerte a ti, pero tengo algo muy importante que decirte. Tu sabes que no te interrumpiria si no fuese completamente necesario".
And here's the English version:
Man: "I was sitting down watching TV and then you came along to talk to me, which distracted me and made me lose attention on what I was watching".
Man 2: "Didn't mean to distract you cousin, but I have something very important to tell you. You know that I wouldn't interrupt you unless it's completely necessary".
The most important thing about "vos" is being able to determine how its used and how it translates into basically "you".