What separates wine from simple juice is the fermentation process in wine making. Fermentation involves wine making yeast, not any other type of yeast like your bread yeast.

Importance
 
Wine yeast is the most important ingredient in wine making. Not only as an ingredient but the process of mixing it with the must will make or break the whole batch of wine. It is said that the real winemaker is not the person making the wine but the yeast itself. The yeast works to absorb the sugar from the fruit and creating alcohol and carbon dioxide in equal portions.
 
Timing
 
The timing of mixing in the yeast is crucial as well. One can neither put the yeast too early nor too late, in the whole wine process. At best, wine making yeast is mixed in at least twenty four hours after the fruit is crushed. And it is left to stand interacting with the must for at least five to seven days, with only a few stirring twice to thrice a day.
 
Activation
 
For better results, wine making yeast must be activated first before mixing it into the fruit must. How? Mix the yeast into a cup of juice squeezed from the must and set aside for at least twelve hours before mixing it into the must. Or a cup of warm water will suffice. Do not use hot water as the temperature will kill the yeast rather than activating it. You know the yeast is active when the mixture looks like it is boiling.
 
Buying
 
Buying yeast is extremely easy. Just make sure that the label reads wine making yeast or something to that fashion. Any generic brand will make do for the amateurs. But if you are the brave type or already an aficionado, those specialty types are great choices. The amount you buy is also noteworthy, a ten milligram packet of yeast is sufficient to work on ten gallons of grape must.
 
So you see, even the hardest part of the wine process, which is the part involving wine yeast, is relatively easy once you know why it is crucial, what to buy and how and when to use it.