Usted/Ustedes - Vosotros/Vosotras
Well, here is something confusing! In order to understand what's the difference between them I had to search in google about a half an hour! haha :) Since I'm interested in traveling in Spain I learn that kind of grammar! So when you are in Spain you use "Usted" when you come across a person that you don't know or it's your teacher or is an older person than you! It's a way to show your respect! "Ustedes" is the same thing but it's not only for one person but for 2 or more. Again we use it to show to the other person our respect! We use "Vosotros/Vosotras" when we are with friends or generally with people that we know and of course there are 2 or more! If i have done (i probably have) some slip up please correct me, I'm on the process of learning any help is valuable!! I know that a lot of you are masters at grammar :)
When conjugating verbs, I find it helpful to use the following table. I can't make a table in the discussion area but I hope you get the gist of it:
Singular - Plural
(first person) Yo - Nosotras/Nosotros
(second person familiar) Tú - Vosotras/Vosotros
(second person formal & third person) Usted/Él/Ella - Ustedes/Ellos/Ellas
Argentina uses "vos" instead of tú so if you ever run across "vos" just realize it's second person singular, same as tú.
You might find this link helpful: http://www.spanishdict.com/topics/show/13 SpanishDict is an excellent resource for Spanish grammar.