Yeast is a very important part of the fermentation process. It is a living organism that feeds on the sugar and converts the sugar in your "must" to alcohol. The cells multiply and stimulate a faster rate of sugar conversion.
The fermentation environment is also key. Growth activity can slow or stop if there is a lack of nutrients, not enough oxygen, too high or low temperature, or too much sulfur dioxide.
Different strains are designed also for different characteristics of a wine. Are you making a dry wine? A light style wine? Aging it in oak barrels? A full-bodied wine? A sparkling wine? Or maybe an icewine? Will you be aging for a long period of time or drinking it early?
Some strains can tolerate higher alcohol levels. Lower alcohol tolerant strains can cause fermentation to slow or even stop, become stuck. So if you have ever had a stuck fermentation and needed to add champagne yeast to it, that is because it can tolerate higher alcohol levels.
When making wine from fresh grapes or fruit, it is essential to have proper nutrients to be sure the yeast can grow and multiply. Most recipes will give you the basic nutrients needed for the type of fruit you are using.
One packet is usually enough for 5 gallons of wine. A good wine yeast will form a firm sediment that settles to the bottom or you primary fermenter to making racking easier, be able to produce at least 10% alcohol, is easy to start, and not create an "off" flavors in your wine.
Bakers yeast will not form a good sediment. They decompose much faster creating "off" flavors in your wine.
Lavlin and Red Star are the most popular brands used today.