The default is, of course, "doc" – common for MS Word documents. To make the file "readable" for previous Word versions (95, 97, NT) – scroll down and choose the corresponding option. By the way, for Word 2007 users – I really recommend to save in Word 97-2003 format instead of "docx" as many people still have no Office 2007 installed.
Another option is "dot" – a document template which will have certain "variables" written to it, which are accessible by other documents. The template can be "locked" for editing to defend your important designs from accidental or deliberate changes.
The "txt" is a basic text format, which is accessible by virtually every text-editor (such as "Notepad"), but these files can only have very simple formatting. So when saving as "plain text" you are almost sure to lose you design.
A more advanced option is "rtf" (Rich Text) format. It is also recognized by many text-editors (even the linux- and mac-based ones). Moreover, it can keep most of your document design – formatting, styles, and so on. Of course, some of it could be lost in the text-editor conversion, but much is kept.
The last group of save formats is the web-pages formats.
The known "html" and "htm" formats are for saving your document as webpage. It is advisable to use the "web-page, filtered" option, as it removes Office-specific tags that are "attached" to your document by Word. Notice that in this option, your pictures and other "insertions" are saved in separate folder. If you want to save everything as one file, you should choose the "single-page" option, "mht" or "mhtm". This will create a larger file, but will keep all data in one place, making the publishing easier.