An avocation is an activity that you pursue when you're not at work — a hobby. Pretty much anything can be an avocation: tennis, sudoku, writing poetry. If you're the journalist Clark Kent, your avocation is changing into a skintight red and blue jumpsuit and fighting crime.
A vocation is the work you do because you have to; an avocation is what you do for pleasure, not pay. The ancient Latin root is a vocare a calling away from one's work, or a distraction. Today we use the word more to refer to a serious hobby. If you like knitting beautiful sweaters, then you, my friend, have an avocation — as long as you're not making your living from it.