Kali Linux is the Next-Generation Backtrack Linux. Or as some people might say,
Backtrack Linux 6.0 = Kali Linux 1.0
So why did they changed the name of the distribution and created the confusion in the first place? The main reason was that the new release of Backtrack (i.e., Kali Linux now onwards) is a very different and advance version . It has many differences as compared to the original Backtrack distribution, and hence, using the same name with new version number would not do justice to their work. Hence they came up with a new name.
Now the question arises, what are the main differences (upgrades) in Kali Linux as compared to the original Backtrack Linux? To know the answer, see what the documentation at Kali Linux says here.