angiedaytripper's answer is partially incorrect:

-- "du" is the informal, singular 2nd person, as in the English "you"

-- "ihr" is the informal, plural 2nd person, as in the Texan "y'all"

-- "Sie" (capital 'S') is the formal singular 2nd person, used in formal contexts, to strangers, co-workers, professors, etc.

-- "sie" is the plural 3rd person, as in the English "they" (as compared to singular 3rd person, "er"/"sie"/"es") -- yes, it can be ambiguous.

My German teacher taught me that Ihr is used the same way that southerners use y'all.

both are informal. think of Du as bro. and ihr as y'all.

Du is informal you, something you would say to friends and peers. Ihr is you guys, everyone in the room but yourself. Sie (capatalized always) is a formal you that one would use when speaking with an elder or say to a professor and shows respect.

'Y'all' is a perfect comparison. Use 'du' for family, friends and people that are younger than you. Use 'sie' for strangers and co-workers and teachers etc. (formal/informal)

Ihr - is the collective 'du (informal you)' and 'Sie' (with a capital 'S') is the collective of 'sie (formal you)'

We used to have these distinctions in English, think 'thee and thou' in Shakespeare, but they're only found in regional dialects now.

Here's a site showing tables for the different personal pronouns and when to use them. They're easy to get confused so it's best to try and learn the rules!

http://www.easygermanlanguage.com/german-pronouns.html

"Du" is for singular, and "ihr" is plural. Take care that "ihr" is also used as "to/for her"as a dative pronoun. (e.g. "Ich danke ihr").

i was about to ask the same question.

I was just about to raise that question.@galliumarsenide is very comprehensive

Ja. Plus, it is "Du unt Ihr"

My (German) mother told me that "Ihr" is formal as is "Sie" and "Du" is informal. Younger people are more lenient so if you stuff up they'll forgive you, but oldies will be offended if you call them "du".

Du hast einen Apfel. - You have an apple. Du und dein Bruder (you and your brother), ihr habt jeder einen Apfel (you have each an apple). If "you" is more then one person then it is in German "ihr" or "Sie". Sie und ihr (your) Buder haben jeder einen Apfel.