You
You is the second person plural pronoun in English. In standard English, it serves as the second person singular pronoun as well.
In modern standard English, you serves as both the nominative and oblique case. The genitive case is your, and the possessive predicate pronoun is yours.
Etymology
It is descended from Old English ge or ȝe, (both pronounced roughly like Modern English yea) which was the old nominative case form of the pronoun, and eow, which was the old accusative case form of the pronoun. In Middle English the nominative case became ye, and the oblique case (formed by the merger of the accusative case and the former dative case) was you. In early Modern English either the nominative or the accusative forms have been generalized in most dialects. Most generalized you; some dialects in the north of England and Scotland generalized ye, or use ye as a clipped or clitic form of the pronoun.
Ye and you are cognate with Dutch jij and jou (gij in dialect or old Dutch), German ihr, and Gothic jus. The specific form of this pronoun is unique to the Germanic languages, but the Germanic forms ultimately do relate to the general Indo-European forms represented by Latin vos.
Note that in the early days of the printing press, the letter y was used in place of the þ, so many modern instances of ye (such as in "Ye Olde … Shoppe") are in fact examples of the and not of you.
Both singular and plural
In standard English, you is both singular and plural; it always takes a verb form that originally marked the word as plural, such as you are. This was not always so.
Old English distinguished between the plural you and the singular thou. This distinction was lost in modern English due to the importation from France of a Romance linguistic feature which is commonly called the T-V distinction. This distinction made the plural forms more respectful and deferential; they were used to address strangers and social superiors. This distinction ultimately led to familiar thou becoming obsolete in standard English. Ironically, the fact that thou is now seen primarily in literary sources such as the King James Bible (often in reference to God) or Shakespeare (often in dramatic dialogs, e.g. "Wherefore art thou Romeo?"), has led many modern anglophones to perceive it as more formal, not familiar (case in point: in Star Wars: The Empire Strikes Back, Darth Vader addresses the Emperor saying, "What is thy bidding, master?").
Because you is both singular and plural, various English dialects have attempted to revive the distinction between a singular and plural you to avoid confusion between the two uses. This is typically done by adding a new plural form; examples of new plurals sometimes seen and heard are you-all/y'all (primarily in the Southeast United States), you guys (Midwest, Northeast, West Coast), youse/youse guys (Australia, New York City region), and you-uns/yins (Pennsylvania). [1] All of these new plurals are marked as basilectal, though they may be useful. English spoken in Ireland, known as Hiberno-English, sometimes uses the word ye as the plural form of you, but more commonly uses youse, which is more commonly spelt as yous.
You is also unusual in that, being both singular and plural, it has two reflexive forms, yourself and yourselves. However, in recent years singular themself is sometimes seen: see singular they.
For a discussion of the alternative "ye" spelling of the definitive article, please see Þ or Ye Olde.
Y'all
In the Southern United States dialects of American English, the term y'all, a contraction of you all, serves as the vernacular second-person plural pronoun. Because, in formal written English, the second-person pronoun you can be both singular and plural, many regional or national dialects have developed an informal distinction. This distinction is often made by adding a word to follow you in order to make a pronominal phrase for the plural. In British English, for example, the function is served by the phrase you lot. In many U.S. dialects, we find you guys (or, quite often, yous(e) guys). Likewise, in the Deep South, the appropriate term is y'all. However, in some areas of the South, including Texas, they are quite proud of this usage and many educated people there consider its usage viable for semi-formal communications, preserving their own distinctive dialect. It is also quite useful when talking with, or translating for, people less familiar with the English language (German and Spanish, for example, having different words for second-person singular and second-person plural).
A case can be made that y'all may have a singular reading as well. For many speakers of ya'll-using dialects, the pronoun may certainly be used in the context of a single person, provided that the addressee is implicitly taken to be part of a larger group. So, for example, a person in a restaurant may ask a waiter "What kind of drinks do y'all have?", but if the question is asking about the waiter's drinks distinct from those of the restaurant, this usage would be ungrammatical. In this respect, this use of y'all is no more unexpected than use of exclusive we when only two individuals are actually physically present.
Partly due to the rising popularity of rap music from the 1980s on, the use of the word "y'all" in the United States has spread from its original regionalism. The syllable has proved useful in keeping metric cadences (as opposed to either "you guys" or "you lot"), yet the single syllable can be stretched-out, allowing the open vowel "aw" to create a much longer-sounding word. The dissapearance of the Anglo-Saxon "thou" and its informal sense has some influence over the seeming need to accept "y'all" into more and more contexts. Ironically, the archaic form of informal English address, "thou," and the popular slang version of informal, second-person do not agree in number: "thou" is only singular, and "y'all" always implies some sense of plurality. This convergence of sociology and liguistics is, actually, quite an important faultline in the continuing shifts of the English language.