EV - Expected Value
This is the fundamental basis of all poker decisions. It can loosely be defined as what your share of the pot would be worth if the hand were replayed millions of times. With a very strong hand, you will win some and lose some, but over time in many situations similar to the current one, you will win lots of money. EV, or equity as its sometimes referred to, is the fundamental factor driving your thought processes on each hand.
Essentially, you want to get more money in the pot when you have more equity (usually that means raising, but sometimes it means calling).
And you want to limit the amount of money going in the pot when you don't have some, but limited, equity (usually that means calling, but occasionally it means folding or raising).
And it means folding when you little equity.
It does NOT mean waiting until you make your hand to put more money in - often it is too late then as your opponents have seen you make your hand and won't pay you off.
What it means in practice is: if you'll win more than your fair share, then do what ever you need to to get more money in. Let's take a look at some hand examples to see this in practice.