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