Vos was the Latin pronoun for "you", in the plural. Some derived words in modern romance languages include:
In Latin, there was no notion of formal versus informal second person pronouns. Tu was "you" and vos was "you guys". By the middle ages however speakers of Romance languages began to use vos in the singular as a formal pronoun. This can be seen today in French, Italian, and Portuguese.
It was also seen for a time in Spanish, where vos was the formal singular pronoun, with vosotros becoming the new informal plural pronoun. Today, the use of vos as a formal singular is archaic, replaced with usted.
However in some parts, mostly in South America, the pronoun vos is still used informally. The use of vos in the place of tú as a singular informal pronoun is called voseo.
In Spanish, the conjugation of the vos pronoun generally looks similar to the vosotros conjugation, although dipthongs may turn into monopthongs: habláis becomes hablás; tenéis becomes tenés; sois becomes sos. For imperative mood, the final d of the vosotros conjugation is not present: tened becomes tené.