Monday's poker guide explains what poker EV is and how to apply the concept in a couple of specific poker situations.
"One of the most used terms in poker is EV, which stands for expected value. In poker, we constantly hear about making good decisions. EV is how one quantifies which are good decisions to make or not. If you make +EV decision, statistics and percentages say that you will make money. In the short run, you may lose the hand due to the nature of luck in poker, but EV is calculating the long term probabilities of what should happen over a limitless sample of hands. If you make a -EV play, you will lose money in the long run. By calculating your EV, you are determining the amount of money you would win or lose on average, on your bet."
The article can be found in the beginner guides sub section of poker guides area of the poker section in the sub tabs.