The basic idea behind StructuredText is to avoid the necessity for a markup language like TeX, Troff or HTML/SGML/XML and being able to create richly structured documents without thinking about the markup. The StructuredText markup language is intiutive, minimal and unobtrusive.
There are several different versions of StructuredText:
reStructuredText - intended primarily for documentation
PyTextile - intended for web use
One application of StructuredText which is currently discussed on the DocSig is the generation of documentation from Python DocStrings.
See TextFilter for more information on text filters, like StructuredText. We have a bit of information about StructuredText mark up.