| In grammar, the second person is you, the reader or the audience. Whereas first is me/we and third is him/them, second person is always you.
You is plural and singular like moose, in English. Other forms:
Nominative (subject of clauses): You
You are reading this node.
Genitive (possessive): Your, Yours
Your eyes look at this node right now. All your base are belong to us. It was yours.
Objective, Ablative (everything else): You, You, You.
There is no "Yous" or "Youse". Do not use them.
Y'all is a contracted form of "You all" and as such is an acceptible usage.
In formal writing, the use of "you" (to speak in general) has been replaced with "one" as in this sentence: One should never use 'youse'. The exception to this is interactive writing, exemplified in choose-your-own-adventure books or Infocom text games.
Older, English second person pronoun substitutions include Thou for the subjective (Thou shalt not kill), Thee for the objective (I smite thee!), and even Ye for the subjective plural (God Bless Ye Merry, Gentlemen).
Many languages differentiate the various forms of "you". For example, in Latin, the endings for singular "you" verbs and plural differed greatly -- "You (sing.) sing" (as to a soloist) is Clamas whereas "You (plur.) sing" (as to a chorus) is Clamatis. This is, of course, different from the imperative and vocative uses, wherein "You" is the one being addressed, often in an understood sense. This also has a singular and plural.
Such as the understood (you) in Stop reading this node!which would apply equally well to singular and plural persons who can read English.
This was a nodeshell rescue, and tempted as I was to put something here about the grassy knoll, my copy editor instincts kicked in too stongly. | 
