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. Early Modern 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, although this did not happen in other languages such as French. Ironically, because thou is now seen primarily in literary sources such as King James Bible (often directed to God, who is traditionally addressed in the familiar) or Shakespeare (often in dramatic dialogs, e.g. »Wherefore art thou Romeo?«), many modern anglophones erroneously perceive it as more formal, rather than familiar (case in point: in Star Wars: The Empire Strikes Back, Darth Vader addresses the Emperor saying, »What is thy bidding, my 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 y'all, or you-all (primarily in the southern United States and African American Vernacular English), you guys (in the U.S., particularly in Midwest, Northeast, and West Coast, in Canada, and in Australia), you lot (in the UK), youse guys (in the U.S., particularly in New York City region, Philadelphia, Michigan's Upper Peninsula and rural Canada; also spelt without the E), and you-uns/yinz (Western Pennsylvania, The Appalachians). English spoken in Ireland, known as Hiberno-English, sometimes uses the word ye as the plural form, or yous (also used in Australia, however not the form ye). Although these plurals are useful in daily speech, they are generally not found in Standard English. Among them, you guys is considered most neutral in the U.S. It is the most common plural form of you in the U.S. except in the dialects with y'all, and has been used even in the White House.
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. abbreviation (used in 'text talk'): u