We can all think of a bad day at work, but can you think of any time when you were "in the zone" - a moment where you enjoyed your task, you were caught up in your work and lost track of time? What were you doing at that moment? What skills were you using? What was it that caused you to be passionate about the work? Understanding the answers to these questions will often help you better understand how to best use your time and talents - a calling.
"With this in mind, we constantly pray for you, that our God may count you worthy of his calling, and that by his power he may fulfill every good purpose of yours and every act prompted by your faith." - 2 Thessalonians 1:12
A calling is when we are passionately using our God-given gifts in a way that pleases Him. We are not talking about your personality profile, but about those God-given gifts listed in the Bible like teaching, counseling, generosity, administration, and prayer, among many others.
When we find ourselves involved with God at work, we also begin to find the work we were meant to do.