Extra credit? What does that mean?

Looking back on it, I remember most teachers who offered extra credit being suspiciously vague about what that meant--unless it was an extra credit question on a quiz.  Then, you knew you'd get one extra credit point.  In other cases, though, they'd usually just say, "You'll get extra credit," and leave it at that.  I always wanted to ask, "What does that mean?  Will I receive an extra five points for doing this assignment?  Ten points?  An extra A or 100% averaged in with my grade?  An extra one percent or two percent tacked on to my average for the quarter or semester?"  In the majority of my classes, I knew how my grade was determined.  The way extra credit factored into a grade was often mysterious, though, and I don't know why.