I just answered this in another thread.

To the best of my knowledge (no jokes, please), pursers are specially trained F/A's who work widebody aircraft. They go through a couple week training and testing (I think it used to be in Cleveland) and if they pass can do International. They can truly be called purser - and have the pin to prove it.

On narrowbody aircraft, there really is no purser, per say (that's cute). They are actually the lead flight attendants and no special training is necessary. Sometimes they bid for this, sometimes they are stuck with it. On these planes, "First flight attendant," Lead flight attendant" and "Purser" are synonymous.