Kamu digunakan dalam percakapan sehari-hari, sedangkan kau kebanyakan digunakan dalam bahasa yang puitis (poem, poetry). atau juga digunakan dalam Bahasa Indonesia dialek daerah tertentu.

There are thousands and thousands of island of Indonesia, "Kamu" is more common, but people from some part of Sumatera, and some others area tend to use "Kau" Both are correct.

actually those two words are similar.. or even the same^^; but "kamu" is used daily and "kau".. errr.. actually i never heard of "kau" anymore, so i suggest to use "kamu" xD