@liw fountain pens specifically meant for calligraphy tend to have broader nibs (or sometimes other properties - special shapes, a softer nib, that sort of thing), but i don't really think of there being a strict dividing line. also, i think sometimes retailers will describe something as a "calligraphy" pen just to sound fancy.
@johns @liw yeah, totally - i have a bunch of inks somewhere around here that are meant for various kinds of dip pens or brushes and would instantly clog a fountain pen.
people do a lot of calligraphy with fountain pens, but it's kind of an overloaded and domain-dependent word.
anyway for entry-level fountain pen buying, i'd mostly just look at "do i want the tip of the nib to be this wide". (for day to day writing, i like something "fine" or "medium" in most makes.)