Many times in literature there are statements like "White/Black is winning" or "White/Black is better". How does one differentiate between the two? How could it be possible for White to be better and not be winning, or be winning and not better? Is there really a difference between the two or are they the same thing?
In the late stages of the game, an advantage can be easily recognized to be enough to win or not. In the earlier stages (the end of the opening, typically), it's a lot harder to tell.
If White is better, and if he manages to obtain something sizeable out of this avantage, then he will win. If Black fights well, he'll still manage to draw, or turn the tables and win.
"White is winning" means that White should be able to FORCE a win with proper play.
"White is better" means that White has a clear advantage, but if Black plays perfectly going forward, he will be able to draw (or win).
It often takes only one more advantage to get from "White is better" to "White is winning."