The Python tutorial talks about it (scroll down a bit until you get to the part about slicing).
The ASCII art diagram will help you get a clear idea on how does this notation work:
 +---+---+---+---+---+---+
 | P | y | t | h | o | n |
 +---+---+---+---+---+---+
 0   1   2   3   4   5   6
-6  -5  -4  -3  -2  -1