Article Review : "The Concept of Opportunity Cost" by Robert Russell

Opportunity cost is what you have to give up to get something.

So one of the keys to thinking like an economist is always remembering that everything has a cost. This may be one reason economists have fewer friends than they otherwise would. Sometimes people are very happy holding on to the naïve view that something is free. We like the idea of a bargain. We don't want to hear about the hidden or non-obvious costs. Thinking about foregone opportunities, the choices we didn't make, can lead to regret. Choosing this college means you can't go to that one. Marrying this person means not marrying that one. Choosing this desert (usually) means missing out on that one. Sometimes, people just want to eat their cake and have it, too, without being reminded that they missed out on a spectacular piece of pie.

The example of this are in college. What's the cost of college? The obvious part of the cost of college is tuition. It's not room and board because those would be incurred anyway. But the opportunity cost includes the foregone wages from the jobs you could have had if you hadn't gone to college. This is one of the reasons we go to college when we're young without any experience in the workplace—our wages are relatively low so the foregone earnings from going to college are lower.

Source : http://www.econlib.org/library/Columns/y2007/Robertsopportunitycost.html