[Vocation] comes from the Latin vocare, to call, and means the work a man is called to by God.
There are all different kinds of voices calling you to all different kinds of work, and the problem is to find out which is the voice of God rather than of Society, say, or the Superego, or Self-Interest.
By and large a good rule for finding out is this. The kind of work God usually calls you to is the kind of work (a) that you need most to do and (b) that the world most needs to have done. If you really get a kick out of your work, you've presumably met requirement (a), but if your work is writing TV deodorant commercials, the chances are you've missed requirement (b). On the other hand, if your work is being a doctor in a leper colony, you have probably met requirement (b), but if most of the time you're bored and depressed by it, the chances are you have not only bypassed (a) but probably aren't helping your patients much either.
Neither the hair shirt nor the soft berth will do. The place God calls you to is the place where your deep gladness and the world's deep hunger meet.
What a joyful and freeing perspective!