1 used with reference to the person or persons addressed or one such person and one or more associated persons.
2 (as int. with a noun) in an exclamatory statement (you fools!).
3 (in general statements) one, a person, anyone, or everyone (it's bad at first, but you get used to it).
Phrases and idioms:
you-all US colloq. you (usu. more than one person). you and yours you together with your family, property, etc. you-know-what (or -who) something or someone unspecified but understood.
Etymology: OE eow accus. & dative of ge YE(1) f. WG: supplanting ye because of the more frequent use of the obj. case, and thou and thee as the more courteous form
Useful english dictionary. 2012.