Some software companies use TextPad as the standard editor for programming interpreted languages.
On the feature-full side of things, two multi-platform editors tend to go head-to-head: vi and Emacs. Some relatively barebones editors are mentioned above. However, once you learn them, those features often pay handsome dividends.
Every Windows installation comes with Notepad, but some excellent replacements to consider are Notepad2, Notepad++, and TextPad.
On Mac, the most popular text editor is BBEdit (of which TextWrangler is a free version). Some editors are platform-independent (they work on more than one operating system), but most are restricted to one. Do you work on Mac? Linux or Unix? Windows? The first criterion by which you should judge the suitability of an editor is whether it works on the platform you use.
Word processors such as MS-Word or Writer include formatting information when they save a file - that is how the program knows to bold certain text and italicize others. A text editor is a program that saves your files without formatting. To program Python, most any text editor will do.