Python is a dynamic object-oriented programming language that can be used for many kinds of software development. It offers strong support for integration with other languages and tools, comes with extensive standard libraries, and can be learned in a few days. Many Python programmers report substantial productivity gains and feel the language encourages the development of higher quality, more maintainable code.
Python runs on Windows, Linux/Unix, Mac OS X, OS/2, Amiga, Palm Handhelds, and Nokia mobile phones. Python has also been ported to the Java and .NET virtual machines.
3.x is the newest branch of Python and the intended future of the language. Guido van Rossum (the original creator of the Python language) decided to clean up Python 2.x properly, with less regard for backwards compatibility than is the case for new releases in the 2.x range. This allowed several aspects of the core language (such as print and exec being statements, integers using floor division) to be adjusted to be easier for newcomers to learn and to be more consistent with the rest of the language. It also allowed later language features (such as iterators) to be applied to older language features (such as the range builtin which returns a list in 2.x, but a memory efficient iterable in 3.x).
The What's New in Python 3.0 document provides a good overview of the major language changes and likely sources of incompatibility with existing Python 2.x code.
However, the broader Python ecosystem has amassed a significant amount of quality software over the years. The downside of breaking backwards compatibility in 3.x is that a lot of that software doesn't work on 3.x yet.