What is 'os' in Spanish? How and when do I use it?

Actually Vosotros IS used in Latin America. It is non-existent in the Caribbean but it is widely used in Argentina, Venezuela, Colombia, and Chile. In fact, Chile does not use "ustedes" at all. You don't have to learn it, actually. If you master the Spanish approved by the Royal Spanish Academy in Spain (they took off vosotros, but it is used casually in other countries), you can understand vosotros without having to learn it. "Do you want to go to the park?" "�quieres ir al parque?" "�quer�s ir al parque?" The third one is used in vosotros form. See how similar they are? Even for the weird ones like "vos sos lindo" you can pretty much understand the basic meaning.