Lots of people, particularly those familiar with the King James Version of the Bible, probably wonder about the difference between "thou" and "you". It's really pretty simple: "thou" is an archaic second-person singular pronoun, and "you" was originally a second-person plural pronoun. "Thou" faded from use in the 16th and 17th centuries (and was, in fact, quint if not archaic even when the KJV was translated) and almost entirely disappeared from use by the 18th century. In it's place, "you" is now used as both the singular and plural second-person pronoun -- sometimes it refers to you individually, and sometimes it refers to you all as a group.
"Thou" was a much more personal and intimate pronoun because it could only refer to a single person. To the best of my knowledge, English is now one of the only languages without a so-called "tu-vous" (T/V) distinction. No one knows why this is the case, but it seems as if the transition started with the upper classes as early as the 13th century. In modern T/V languages, such as French, I understand that it's considered more respectful to use the V pronoun; T is used in intimate situations or by superiors to inferiors. (Is this right?)
For more information, look up the work by linguist Dick Leith.