Python comes in two distinct versions, namely Python2 and Python3. Unfortunately for Python programmers Python3 is not backwardly compatible with Python2 source code.
The Python2 flavour - currently fixed at version 2.7 - is the most widely used and supported version. Many Linux distributions and all Mac OS X operating systems install Python2 by default, and the latest 'Wheezy' Raspberry Pi distribution includes both Python2 and Python3. In addition, a significant proportion of open-source, community-developed and third-party modules are based on Python2 code.
Many Python coders have both versions installed on their development machines. After all, Python3 has some highly desirable features, most notably full Unicode support. However, care must be taken to ensure any Python2 code doesn't break a Python3 app, and vice versa.
At some point in the future everyone may decide Python3 is the de facto standard - but we haven't reached that point just yet. The Python wiki has plenty more information on the Python2 vs Python3 question.
A post from my Learn Python on the Raspberry Pi tutorial.