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Y'all

Approximate distribution of ya'll land (source)

There are currently four generally recognized properties that "y'all" follows:

1. A replacement for the plural of you.
   • Example: "Y'all can use the internet at the same time!"
2. An associative plural, including individuals associated but not present with the singular addressee.
   • Example: "Y'all can come over at around 10:30," Stephanie says.
     • Stephanie explains to John that John and John's friends, who are not present at the time, can come over at around 10:30. Stephanie is speaking to John, but treats John as a representative for others.
3. An institutional plural addressed to one person representing a group.
   • Example: "Y'all sell the best candies, Mrs. Johnson."
     • Y'all is received by Mrs. Johnson who is the representative of a small candy business.
4. A form used in direct address in certain contexts (e.g., partings, greetings, invitations, and vocatives)
   • Example: "Hey, y'all!"
     • A greeting that addresses a multitude of people without referencing a singular identity comprising that multitude

From Wikipedia: http://en.wikipedia.org/wiki/Y'all
See also: http://en.wikipedia.org/wiki/Southern_American_English