Professionals vs. Experts

It doesn't take a lot to call yourself a professional these days.  In fact, any band or musician that has gotten paid to play can call themselves a professional (and they do... often).  So what's the difference between a reliable/quality group and some college buddies that got paid $50 once to play at their grandmother's birthday party? 1.) I meant to do that! Experienced musicians have been at it a long time; they know how to react...