Other forms: thy; thee; ye; thine; thous
The word thou, used in place of "you," is not used much in modern language. In fact, with its Biblical feeling, it’s most often used in religious contexts. Otherwise, it might be used as slang for thousand.
Anyone familiar with the Bible knows the word thou means "you." As the word worked its way from Old English, it varied a little in its subtext, from a plural referring to higher-ups, then to equals, and, around 1450, as a slightly insulting reference suggesting inferiority. Today, it's used primarily by specific religious groups, or as a somewhat affected, archaic word in poetry. As a side note, if someone asks you for a thou, they mean a thousand dollars.