Python 3 (formerly Python 3000) is a reboot of Python syntax created to address various architectural problems in Python 2 that could not be fixed without breaking backwards compatibility.
Python creator Guido van Rossum proposed Python 3 in PEP 3000 (now moved to PEP 3100) as a list of things that Python developers (ie. the Python language developers, not developers in Python) would like to see but which would break backwards compatibility.
Python 3 was released in December 2008 but at the time of writing (April 2010) is still seeing slow adoption. Though a 2to3 tool exists to help perform one-off translation from Python 2 to Python 3, supporting both in an ongoing fashion requires maintaining both codebases. Thus many Python libraries are unavailable for Python 3, which prevents developers from using Python 3, which prevents library developers bothering to support Python 3.