It's the plural of "tú" (second person singular). Tú eres americano = You are American (singular) Vosotros sois americanos = You are American(s) (plural) In Spain, the words "tú" and "usted" are used when addressing people. Which one you should use depends on the situation. Their plurals are "vosotros" and "ustedes". I tend to use "usted" in formal situations or where politeness is required, or with older people. In most other cases I use "tú". When addressing a group of friends I would usually use "vosotros".

Hello, How are you today, My name is miss Esther, i saw your contact today at (www.italki.com) and become interested to know more about you with all pleasure and i want you to send mail to my e-mail address ([email protected]) so i can give you my picture and tell you more about myself,Waiting for your reply to my email address soon. Have a nice day, Yours Esther. [email protected]

Thank you for your choice and a Happy new year.

its the same than ustedes, solo que con vosotros tenes que usar vosotros tenes que cambiar los verbos como los dicen en España, vosotros trabajáis, ustedes trabajan.