{\raggedright % declaration for ragged text
Here's text to be ranged left in our output,
but it's the only such paragraph, so we now
end the group.}
Here's more that needn't be ragged...
TeX will open a group, and impose the ragged-setting parameters within
that group; it will then save a couple of sentences of text and
close the group (thus restoring the previous value of the
parameters that \raggedright set). Then TeX encounters a blank
line, which it knows to treat as a \par token, so it typesets the
two sentences; but because the enclosing group has now been closed,
the parameter settings have been lost, and the paragraph will be
typeset normally.
end the group.\par} Here's more that needn't be ragged...
In this way, the paragraph is completed while \raggedright’s
parameters are still in force within the enclosing group.
\begin{flushleft}
Here's text to be ranged left...
\end{flushleft}
In fact, there are a number of parameters for which TeX only maintains one value per paragraph. A tiresome one is the set of upper case/lower case translations, which (oddly enough) constrains hyphenation of mutilingual texts. Another that regularly creates confusion is
\baselineskip.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=paraparam