Wikipedia
Voseo is the use of vos as a second person singular pronoun, including its conjugational verb forms in many dialects of Spanish. In dialects that have it, it is used either instead of tú, or alongside it. Use of "tú" is known as "tuteo".
Vos is used extensively as the primary form of the second person singular in Rioplatense Spanish ( Argentina and Uruguay), Eastern Bolivia, and Paraguayan Spanish.
The Central American Spanish ( El Salvador, Guatemala, Honduras, Nicaragua, Costa Rica, southern parts of Chiapas in Mexico) also exhibits an extensive use of vos, though in Northern Central America the media use tú more frequently. In Nicaragua and Costa Rica, tú is used less frequently.
Vos had not been traditionally used in formal writing except in Argentina, Paraguay and Uruguay. This gradually changed in Central America, where even the most prestigious media networks and press began to use the pronoun vos, reflecting the informal address in Spanish as opposed to the formal address of usted. Nowadays it is very common to see billboards and other advertising media using voseo. In the dialect of Argentina, Paraguay and Uruguay (known as 'Rioplatense Spanish'), vos is also the standard form for use in television media.
Vos is present in other countries as a regionalism, for instance in the Maracucho Spanish of Zulia State, Venezuela (see Venezuelan Spanish), in the Azuero peninsula of Panama, in various departments in Colombia, and in parts of Ecuador ( Sierra down to Esmeraldas). In Peru, voseo is present in some Andean regions and Cajamarca, but the younger generations have ceased to use it excepting part of Quito where it is usually found as an informal pronoun. It is also present in Ladino (spoken by Sephardic Jews throughout Israel, Turkey, the Balkans, Morocco, Latin America and the United States), where it replaces usted. In the United States, Salvadoran Americans are by far the largest voseo users followed by other Central Americans, Hondurans, Nicaraguans, and Costa Ricans.
Voseo can also be found in the context of using verb conjugation for vos with tú as the subject pronoun ( verbal voseo), as in the case of Chilean Spanish, where this form coexists with the ordinary form of voseo.