if (is_bbpress()) :
div class=”abc”
else :
div class=”xyz”
endif;translates to
if you are on a bbpress screen/page/display [as opposed to a wordpress blog post or page, a home page or anything that isn't bbpress]
the use div class abc
if you are on any other type of page use class xyz.
Many themes use if statements checking against ‘conditional tags’ to style areas for instance the conditional tag is_home() checks whether this is the home or index page.